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