R1:7b6fa0db12cc

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…