Implement a user profile image cache

Summary:
Ref T7707. The general form of this can probably be refined somewhat over time as we have more use cases.

I put this cache on the user object itself because we essentially always need this data and it's trivial to invalidate the cache (we can do it implicilty during reads).

Also fix an issue with short, wide images not thumbnailing properly after recent changes.

Test Plan:

• Loaded some pages; saw caches write; saw good pictures.
• Reloaded; saw cache reads; saw good pictures.
• Changed profile picture; saw immediate update.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7707

Differential Revision: https://secure.phabricator.com/D12826