Restore old "author can not be a reviewer" rule to Transactions

Summary:
This is a bit messy, but not tooo bad:

  - In general, stop the author from being added as a reviewer.
  - In the specific case that "self accept" is enabled, allow it. This is easier than trying to special case it.
  - When commandeering, we make the author a reviewer and make the actor the author, but these happen after validation. At validation time, it looks like we're making the author a reviewer. Just special case this.
  - Provide a slightly nicer message when trying to add yourself from `arc`. We hit the Transactions message anyway, but it's not formatted as cleanly.
  - Don't try to add the author via Herald.

Test Plan:
  - Edited a revision with author = reviewer, got stopped.
  - Commandeered revision.
  - Updated from `arc`.
  - Updated in general.
  - Fired a "add author as reviewer" Herald rule without things breaking.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aran, epriestley

Differential Revision: https://secure.phabricator.com/D8496