Create SocialAuthMixin generic class around GitHub auth backend.
Create SocialAuthMixin generic class around GitHub auth backend.

This will simplify the process of adding new social authentication
backends to Zulip.

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