Genericize transactions in Pholio

Summary:
Split Pholio's transaction implementation into generic and application-specific parts. Moves us toward generic transactions, with support for:

• Editing and deleting comments.
• Setting visibility of individual comments (I'm not a fan of this feature but we'll see).

I want to move everything to a more generic piece of infrastructure but there's very little they can share right now so adding transactions to, e.g., Paste or Macros (T2157) means massive amounts of similar code.

Tons of work left to do here, but I think it basically works. Here's a screenshot:

{F26820}

Test Plan: Made transactions in Pholio.

Reviewers: btrahan, vrana, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2104

Differential Revision: https://secure.phabricator.com/D4136