Change/drop/reconcile some miscellaneous edit behaviors in Maniphest

Summary:
Ref T9132. Open to discussion here since it's mostly product stuff, but here's my gut on this:

• Change Maniphest behavior to stop assigning tasks if they're unassigned when closed. I think this behavior often doesn't make much sense. We'll probably separately track "who closed this" in T4434 eventually.
• Only add the actor as a subscriber if they comment, like in other applications. Previously, we added them as a subscriber for other types of changes (like priority and status changes). This is more consistent, but open to retaining the old behavior or some compromise between the two.
• Retain the "when changing owner, subscribe the old owner" behavior.

Test Plan:

• Added a comment, got CC'd.
• Changed owners, saw old owner get CC'd.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9132

Differential Revision: https://secure.phabricator.com/D14670