Show small author portraits in Diffusion blame view

Summary: Depends on D18746. See PHI174. Adds small author portraits next to each blame line (this is similar to GitHub).

Test Plan:
My local test data isn't that great since I don't have commits from a lot of accounts, but looks functional:

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18747