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