R1:b1c42f489357

Simplify CC handling in Maniphest

Summary: This fixes a bug where doing an "Add CC" on a task you were not CC'd on
would remove all the CCs except yourself. It also simplifies the CC handling
code a lot.
Test Plan:
- Added myself and another user to a task neither of us were CC'd on (old
behavior: added both then removed them; new behavior: added both)
- Added a user to CC with @mentions.
- Made a comment on a task I wasn't CC'd on (I was CC'd).
- Closed a task I wasn't assigned or CC'd on (I was…