R1:77b23035c3c2

Allow Maniphest to process multiple commands via email

Summary:
Ref T7199. Two notable changes:

- Process multiple commands.
- Process commands when creating //or// updating a task.

And generally clean things up a bit.

Test Plan:
- Used `receive-test` to execute all commands for new tasks.
- Used `receive-test` to execute all commands for existing tasks.
- Used a combination of commands to produce varied effects.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest…