This is the first step in refactoring the infoboxes to use backbone models and

views. The code for user and bug infoboxes was substantially similar, and

there's no reason why they had to be separate. All of the code is now shared,

and the API entry-points (bug_infobox and user_infobox) just call into the

common method and pass an ID for the infobox element.

Tested that both infoboxes worked as expected.