Migrate Pholio image description and mock status to modular transactions

Summary: Also removes more now-dead code from PholioTransaction.

Test Plan: opened and closed a bunch of mocks, edited a bunch of image descriptions

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D17868