In this cleanup I make it so that all jQuery selector references
are toward the top of the module, and we do all finds relative
to the container ('#user_presences').
This will make it easier to make a better list abstraction for
the buddy list, for things like progressive rendering.
This change makes a common code path for these two operations:
* clicking on a user
* hitting enter when a user is highlighted
The newer codepath, for the enter key, had some differences that
were just confusing. For example, there's no need to open the
compose box, since that's already handled by the narrowing code.
For possibly dubious reasons, I let each handler still call
popovers.hide_all() on its own, since it makes the code a bit
more consistent with existing code patterns.
