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