Make @mentions add CCs as a side effect

Summary:
When a user gets @mentioned in Differential, add them as a CC.

No Maniphest hookup yet since I want to make that one a little more formal.

Depends on D518.

Test Plan:
@mentioned a user and they were added as a CC.

Reviewed By: jungejason
Reviewers: tomo, mroch, jsp, jungejason, aran, tuomaspelkonen
CC: aran, jungejason
Differential Revision: 519