Correct self-review check in Differential

Summary: This check is currently wrong -- the actor is only coincidentally the owner (and only most of the time). It also raises at parse time, preventing any user from parsing a message with their own name in the "Reviewers" field. Instead, check against the right owner PHID and raise it only if a revision is available. See https://github.com/facebook/arcanist/issues/54 and next diff.

Test Plan: Tried to add myself as a reviewer to revisions I own via web and Conduit, got rejected. Parsed a message with myself in the "Reviewers:" field, it worked correctly.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D3820