Add extensibility to the user infobox.

Review Request #1370 — Created Sept. 3, 2018 and updated — Latest diff uploaded

Review Board
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

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

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