export: Refactor how we fetch attachment data.
export: Refactor how we fetch attachment data.

This commit doesn't change any behavior; it just moves fetching
attachments out of the Config scheme and into its own method.
This prepares us to start writing attachment data to its own
file and cross-checking against message ids (coming soon).

zacps/zulip,ryanbackman/zulip,amanharitsh123/zulip,jackrzhang/zulip,blaze225/zulip,arpith/zulip,jrowan/zulip,joyhchen/zulip,cosmicAsymmetry/zulip,timabbott/zulip,isht3/zulip,TigorC/zulip,hackerkid/zulip,susansls/zulip,vikas-parashar/zulip,KingxBanana/zulip,verma-varsha/zulip,jphilipsen05/zulip,paxapy/zulip,vikas-parashar/zulip,Jianchun1/zulip,cosmicAsymmetry/zulip,sup95/zulip,sharmaeklavya2/zulip,tommyip/zulip,krtkmj/zulip,grave-w-grave/zulip,calvinleenyc/zulip,sharmaeklavya2/zulip,timabbott/zulip,ryanbackman/zulip,isht3/zulip,ryanbackman/zulip,ahmadassaf/zulip,mohsenSy/zulip,JPJPJPOPOP/zulip,eeshangarg/zulip,dawran6/zulip,showell/zulip,JPJPJPOPOP/zulip,amyliu345/zulip,KingxBanana/zulip,rht/zulip,joyhchen/zulip,blaze225/zulip,rishig/zulip,blaze225/zulip,reyha/zulip,umkay/zulip,jrowan/zulip,dawran6/zulip,susansls/zulip,vaidap/zulip,Jianchun1/zulip,reyha/zulip,samatdav/zulip,vikas-parashar/zulip,isht3/zulip,rishig/zulip,eeshangarg/zulip,Jianchun1/zulip,sharmaeklavya2/zulip,zulip/zulip,brainwane/zulip,samatdav/zulip,joyhchen/zulip,sharmaeklavya2/zulip,mahim97/zulip,j831/zulip,sup95/zulip,niftynei/zulip,brainwane/zulip,aakash-cr7/zulip,rht/zulip,brockwhittaker/zulip,aakash-cr7/zulip,vaidap/zulip,dattatreya303/zulip,showell/zulip,christi3k/zulip,showell/zulip,brainwane/zulip,verma-varsha/zulip,mohsenSy/zulip,calvinleenyc/zulip,TigorC/zulip,zulip/zulip,peguin40/zulip,jainayush975/zulip,vabs22/zulip,vaidap/zulip,amyliu345/zulip,PhilSk/zulip,andersk/zulip,jainayush975/zulip,amyliu345/zulip,dattatreya303/zulip,SmartPeople/zulip,christi3k/zulip,amanharitsh123/zulip,calvinleenyc/zulip,susansls/zulip,Galexrt/zulip,susansls/zulip,rht/zulip,dattatreya303/zulip,vaidap/zulip,synicalsyntax/zulip,arpith/zulip,blaze225/zulip,umkay/zulip,vabs22/zulip,mahim97/zulip,jrowan/zulip,punchagan/zulip,paxapy/zulip,AZtheAsian/zulip,amyliu345/zulip,vikas-parashar/zulip,brainwane/zulip,mohsenSy/zulip,aakash-cr7/zulip,brockwhittaker/zulip,krtkmj/zulip,reyha/zulip,rht/zulip,punchagan/zulip,dhcrzf/zulip,Diptanshu8/zulip,grave-w-grave/zulip,dhcrzf/zulip,JPJPJPOPOP/zulip,ahmadassaf/zulip,dawran6/zulip,dhcrzf/zulip,reyha/zulip,niftynei/zulip,rishig/zulip,brockwhittaker/zulip,AZtheAsian/zulip,aakash-cr7/zulip,brockwhittaker/zulip,sharmaeklavya2/zulip,dawran6/zulip,tommyip/zulip,peguin40/zulip,souravbadami/zulip,KingxBanana/zulip,jrowan/zulip,umkay/zulip,jphilipsen05/zulip,TigorC/zulip,andersk/zulip,arpith/zulip,jphilipsen05/zulip,paxapy/zulip,paxapy/zulip,mohsenSy/zulip,jphilipsen05/zulip,umkay/zulip,verma-varsha/zulip,amyliu345/zulip,sup95/zulip,tommyip/zulip,jackrzhang/zulip,sonali0901/zulip,rishig/zulip,verma-varsha/zulip,christi3k/zulip,punchagan/zulip,KingxBanana/zulip,synicalsyntax/zulip,ryanbackman/zulip,zacps/zulip,arpith/zulip,TigorC/zulip,brockwhittaker/zulip,vabs22/zulip,Jianchun1/zulip,dawran6/zulip,umkay/zulip,brainwane/zulip,rht/zulip,grave-w-grave/zulip,vabs22/zulip,kou/zulip,rht/zulip,grave-w-grave/zulip,zacps/zulip,niftynei/zulip,showell/zulip,tommyip/zulip,kou/zulip,Juanvulcano/zulip,peguin40/zulip,tommyip/zulip,j831/zulip,Juanvulcano/zulip,shubhamdhama/zulip,souravbadami/zulip,shubhamdhama/zulip,grave-w-grave/zulip,sonali0901/zulip,j831/zulip,SmartPeople/zulip,blaze225/zulip,synicalsyntax/zulip,souravbadami/zulip,dawran6/zulip,peguin40/zulip,hackerkid/zulip,jackrzhang/zulip,punchagan/zulip,isht3/zulip,krtkmj/zulip,SmartPeople/zulip,zulip/zulip,Juanvulcano/zulip,PhilSk/zulip,arpith/zulip,amanharitsh123/zulip,hackerkid/zulip,Galexrt/zulip,joyhchen/zulip,jrowan/zulip,kou/zulip,vaidap/zulip,umkay/zulip,jainayush975/zulip,AZtheAsian/zulip,kou/zulip,mahim97/zulip,christi3k/zulip,peguin40/zulip,cosmicAsymmetry/zulip,KingxBanana/zulip,aakash-cr7/zulip,andersk/zulip,samatdav/zulip,christi3k/zulip,reyha/zulip,Juanvulcano/zulip,punchagan/zulip,TigorC/zulip,vikas-parashar/zulip,mahim97/zulip,verma-varsha/zulip,Juanvulcano/zulip,dhcrzf/zulip,PhilSk/zulip,ahmadassaf/zulip,niftynei/zulip,showell/zulip,dhcrzf/zulip,jainayush975/zulip,kou/zulip,synicalsyntax/zulip,hackerkid/zulip,aakash-cr7/zulip,souravbadami/zulip,Galexrt/zulip,eeshangarg/zulip,peguin40/zulip,shubhamdhama/zulip,SmartPeople/zulip,SmartPeople/zulip,isht3/zulip,sonali0901/zulip,reyha/zulip,souravbadami/zulip,cosmicAsymmetry/zulip,samatdav/zulip,calvinleenyc/zulip,calvinleenyc/zulip,niftynei/zulip,zulip/zulip,PhilSk/zulip,dhcrzf/zulip,ahmadassaf/zulip,brainwane/zulip,krtkmj/zulip,eeshangarg/zulip,JPJPJPOPOP/zulip,synicalsyntax/zulip,jphilipsen05/zulip,zulip/zulip,Jianchun1/zulip,shubhamdhama/zulip,christi3k/zulip,ahmadassaf/zulip,dattatreya303/zulip,eeshangarg/zulip,isht3/zulip,shubhamdhama/zulip,rishig/zulip,KingxBanana/zulip,rishig/zulip,amyliu345/zulip,Galexrt/zulip,cosmicAsymmetry/zulip,dhcrzf/zulip,punchagan/zulip,blaze225/zulip,krtkmj/zulip,susansls/zulip,showell/zulip,mahim97/zulip,sup95/zulip,AZtheAsian/zulip,Diptanshu8/zulip,zacps/zulip,rishig/zulip,PhilSk/zulip,souravbadami/zulip,dattatreya303/zulip,amanharitsh123/zulip,Diptanshu8/zulip,arpith/zulip,amanharitsh123/zulip,punchagan/zulip,brainwane/zulip,rht/zulip,krtkmj/zulip,grave-w-grave/zulip,mahim97/zulip,sonali0901/zulip,jainayush975/zulip,jackrzhang/zulip,susansls/zulip,paxapy/zulip,Diptanshu8/zulip,hackerkid/zulip,verma-varsha/zulip,Galexrt/zulip,synicalsyntax/zulip,cosmicAsymmetry/zulip,zacps/zulip,sonali0901/zulip,jainayush975/zulip,joyhchen/zulip,timabbott/zulip,sonali0901/zulip,JPJPJPOPOP/zulip,sharmaeklavya2/zulip,jphilipsen05/zulip,ryanbackman/zulip,ahmadassaf/zulip,zulip/zulip,andersk/zulip,joyhchen/zulip,brockwhittaker/zulip,eeshangarg/zulip,TigorC/zulip,ahmadassaf/zulip,krtkmj/zulip,Galexrt/zulip,kou/zulip,synicalsyntax/zulip,tommyip/zulip,showell/zulip,JPJPJPOPOP/zulip,andersk/zulip,hackerkid/zulip,Jianchun1/zulip,jackrzhang/zulip,timabbott/zulip,vaidap/zulip,PhilSk/zulip,hackerkid/zulip,vabs22/zulip,amanharitsh123/zulip,timabbott/zulip,zacps/zulip,AZtheAsian/zulip,andersk/zulip,jrowan/zulip,mohsenSy/zulip,eeshangarg/zulip,Diptanshu8/zulip,vikas-parashar/zulip,timabbott/zulip,Galexrt/zulip,j831/zulip,SmartPeople/zulip,umkay/zulip,samatdav/zulip,sup95/zulip,andersk/zulip,zulip/zulip,paxapy/zulip,dattatreya303/zulip,calvinleenyc/zulip,Diptanshu8/zulip,Juanvulcano/zulip,j831/zulip,samatdav/zulip,AZtheAsian/zulip,sup95/zulip,ryanbackman/zulip,mohsenSy/zulip,shubhamdhama/zulip,jackrzhang/zulip,vabs22/zulip,tommyip/zulip,niftynei/zulip,timabbott/zulip,j831/zulip,shubhamdhama/zulip,jackrzhang/zulip,kou/zulip