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 Tasks: T7199

Differential Revision: https://secure.phabricator.com/D12234