The new is obviously parallel with the small avatar URL construction,
and allows us to deduplicate this construction between the popovers
and full user profile logic for getting a medium avatar URL.
Fixes#20140.
We add unsubscribe button in the stream list that allows
users and admins to unsubscribe them using profile modal.
If a user attempts to unsubscribe from a private stream
we redirect them to do so from stream setting overlay
to avoid a really confusing user experience as we ask
for confirmation before unsubscribing such streams.
We had a lot of functions and click handlers that were only
involved with user profile modal and were not related to
popovers logic in any way. So we extract these functions
into a separate module `user_profile.js`.