Make maniphest add CCs when users are @mentioned

Summary:
We don't currently add CCs, but should (similar to how Differential works).

This also fixes some problems where you can get no-op CC transactions, and makes
mentions a little less aggressively colored.

Test Plan: Applied a bunch of CC/mention transactions to tasks and observed the
behavior.
Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran, codeblock
CC: aran, jungejason
Differential Revision: 634