R1:8065433ee8dc

Migrate DiffusionBlameController to use repo identities

Summary:
Now on the blame page, identities get `avatar.png` and there are little tooltips that show a few characters of the committer identity string.

Also add a default icon for repo identities.

Test Plan: Loaded some blame pages for files touched by users with and without repo identities attached.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D19587