Migrate Project image to modular transactions

Summary: I'm not sure you can actually remove a project's image (maybe via the API?), but I kept the code for rendering the relevant title/feed anyway.

Test Plan: Unit tests + adding/changing project pictures.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D17954