refactor: Move code for can't-subscribe popover.
refactor: Move code for can't-subscribe popover.

kou/zulip,eeshangarg/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,andersk/zulip,kou/zulip,kou/zulip,rht/zulip,zulip/zulip,kou/zulip,rht/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,rht/zulip,rht/zulip,zulip/zulip,kou/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,andersk/zulip