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 not CC'd, but was

assigned).

• Made an "Add CC" with new CCs and comment text (ccs added, text appeared).
• Made an "Add CC" with no CCs and comment text (text appeared, transaction

correctly downgraded to "comment").

• Made an "Add CC" with exsiting CCs and comment text (text appeared,

transaction correctly downgraded to "comment").

Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, jungejason
Differential Revision: 668