Add extensibility to the user infobox.
Review Request #1370 — Created Sept. 3, 2018 and updated
This change adds a new extension hook, the UserInfoBoxHook, which allows extensions to inject additional data into the infobox. This is shown in a new panel below the existing information, so it doesn't screw up the existing layout. This also fixes up a couple bugs in my earlier redesign change that emerged as I tested it with this. Based on work by Chronicle Yu at https://reviews.reviewboard.org/r/7662/ Testing done: - Built a simple extension which lists a count of the users' incoming and outgoing review requests in the infobox and saw the correct results. - Ran unit tests. Reviewed at https://reviews.reviewboard.org/r/8078/