Add an "Add CCs" action to Differential

Summary:
We currently have only an "Add reviewers" action, add "Add CCs". This can also
be accomplished less-discoverably with mentions.

Test Plan:
Added reviewers and CCs to revisions. Toggled display between reviewers and CCs.

Reviewed By: jungejason
Reviewers: tomo, mroch, jsp, jungejason, aran, tuomaspelkonen
CC: aran, jungejason
Differential Revision: 521