Change Pholio editor access modifier

Summary: Used by PholioImageFileTransaction::mergeTransactions(). I forgot to test adding multiple images to a Mock at the same time after migrating mergeTransactions over to the modular framework.

Test Plan: Added multiple images in a single transaction and didn't get an exception about accessing a protected function.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D17946