Add profile images to Repositories

Summary: Builds out some images to use to identify repositories. Fixes T12825.

Test Plan:
Try setting custom, built in, and null images.

{F4998175}

{F4998192}

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12825

Differential Revision: https://secure.phabricator.com/D18116