Implement the "!priority" and "!status" mail commands

Summary: Ref T7199. Adds "!priority" and "!status".

Test Plan:

• Used !priority and !status to adjust tasks.
• Changed config and provided keywords.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7199

Differential Revision: https://secure.phabricator.com/D12248