R1:df264d85484a

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…