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.

• 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.