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