When hovering over a user's name, an info box appears showing their avatar/photo, name, e-mail address, and some other information. It's somewhat useful, but could be more useful.

This change adds a couple new pieces of information:
1) Their timezone and how many hours ahead/behind you they are (to help determine if they'd be available at the moment).

2) Some review request stats (how many review requests they have been assigned to, and how many they have had out for review).
A new UserInfoExtensionHook is also created to provide additional information, and associated entity tag data, for this box, should the need arise. That way, third-parties can integrate other services they use with this box. Documentation and example usage for the new hook are also provided.

Moused over usernames to verify that the infobox appears with the new (and old) pieces of information displays correctly.

Overrode get_extra_context to ensure that additional information added via the extension hook displays correctly.