R1:f1744ac6d9f8

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…