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`.
  - Added auditors.
  - Accepted a commit.

Reviewers: joshuaspence, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4896

Differential Revision: https://secure.phabricator.com/D10109