Make Pholio mock edits (mostly) transaction-oriented

Summary:

• Use transactions to apply edits.
• Use Editor to apply transactions.
• Some special casing for tricky stuff I don't want to deal with yet (mock images).

Test Plan: {F22368}

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2097

Differential Revision: https://secure.phabricator.com/D3837