forms.HomepageForm: Consolidate check for open realm.
forms.HomepageForm: Consolidate check for open realm.

Does change/fix behavior in various corner cases when the domain passed in
to HomepageForm and subdomain passed in to HomepageForm correspond to
different realms.

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