Pholio - support editing images - fixes T3489

Summary:
Nice title. We add three new transactions - IMAGE_FILE, IMAGE_NAME, and IMAGE_DESCRIPTION. The first is a bit like subscribers as it is a list of file phids. The latter have values of the form ($file_phid => $data), where $data is $name or $description respectively. This is because we need to collate transactions based on $file_phid...

Overall, this uses the _underyling files_ and not the "PholioImage" to determine if things are unique or not. That said, simply mark PholioImages as obsolete so inline comments about no-longer applicable PholioImages don't break.

Does a reasonable job implementing the mock. Note you can't "update" an image at this time, though you can delete and add at will.

Test Plan: played with pholio a ton.

Reviewers: epriestley

Reviewed By: epriestley

CC: chad, aran, Korvin

Maniphest Tasks: T3489

Differential Revision: https://secure.phabricator.com/D6441