R1:5b969fb5b809

Provide a transaction editor to perform Audit row writes

Summary:
Ref T4896. Replaces more custom stuff with standard stuff. In particular:

- No more fake proxy writes;
- no more fake detection of `@mentions`.

For now, the old code still applies most of the effects and handles feed and email.

Test Plan:
- Added comments.
- Added comments with inline comments.
- Added just inline comments.
- Added comments with Conduit.
- Previewed comments.
- Added CCs explicitly and with `@mentions`.