auth: Factor out some uses of EXTERNAL_HOST with ROOT_DOMAIN_URI.
auth: Factor out some uses of EXTERNAL_HOST with ROOT_DOMAIN_URI.

Apart from being less verbose, this makes it more manifest (on e.g. grep)
that we aren't using EXTERNAL_HOST here to construct subdomains.

zulip/zulip,shubhamdhama/zulip,eeshangarg/zulip,andersk/zulip,synicalsyntax/zulip,Galexrt/zulip,jackrzhang/zulip,showell/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,shubhamdhama/zulip,timabbott/zulip,timabbott/zulip,kou/zulip,brockwhittaker/zulip,zulip/zulip,shubhamdhama/zulip,hackerkid/zulip,tommyip/zulip,jackrzhang/zulip,mahim97/zulip,Galexrt/zulip,timabbott/zulip,Galexrt/zulip,dhcrzf/zulip,Galexrt/zulip,showell/zulip,tommyip/zulip,brainwane/zulip,dhcrzf/zulip,synicalsyntax/zulip,synicalsyntax/zulip,shubhamdhama/zulip,hackerkid/zulip,rht/zulip,Galexrt/zulip,mahim97/zulip,jackrzhang/zulip,synicalsyntax/zulip,rht/zulip,dhcrzf/zulip,rht/zulip,synicalsyntax/zulip,timabbott/zulip,kou/zulip,synicalsyntax/zulip,dhcrzf/zulip,zulip/zulip,punchagan/zulip,zulip/zulip,kou/zulip,brockwhittaker/zulip,hackerkid/zulip,dhcrzf/zulip,showell/zulip,dhcrzf/zulip,rishig/zulip,Galexrt/zulip,shubhamdhama/zulip,showell/zulip,hackerkid/zulip,mahim97/zulip,rishig/zulip,jackrzhang/zulip,eeshangarg/zulip,hackerkid/zulip,rishig/zulip,punchagan/zulip,rishig/zulip,brainwane/zulip,brainwane/zulip,jackrzhang/zulip,rishig/zulip,hackerkid/zulip,andersk/zulip,zulip/zulip,andersk/zulip,tommyip/zulip,brockwhittaker/zulip,punchagan/zulip,eeshangarg/zulip,rht/zulip,tommyip/zulip,jackrzhang/zulip,timabbott/zulip,brainwane/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,brockwhittaker/zulip,tommyip/zulip,brainwane/zulip,kou/zulip,rishig/zulip,dhcrzf/zulip,timabbott/zulip,andersk/zulip,showell/zulip,eeshangarg/zulip,punchagan/zulip,mahim97/zulip,andersk/zulip,brockwhittaker/zulip,showell/zulip,jackrzhang/zulip,zulip/zulip,punchagan/zulip,rishig/zulip,brainwane/zulip,eeshangarg/zulip,mahim97/zulip,shubhamdhama/zulip,tommyip/zulip,timabbott/zulip,mahim97/zulip,showell/zulip,rht/zulip,shubhamdhama/zulip,tommyip/zulip,Galexrt/zulip,andersk/zulip,punchagan/zulip,brockwhittaker/zulip,synicalsyntax/zulip,brainwane/zulip,kou/zulip,hackerkid/zulip,punchagan/zulip