CC users mentioned in revision fields

Summary:
Users are used to this feature from comments.
Provide it also in title, summary and test plan.
It adds the users to CC only on creating the revision to avoid cases like:
"I mentioned this user but now I want to remove him from CC" or he unsubscribes.

Test Plan: Wrote @epriestley to summary.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4050