R1:52eab8760870

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.

{F355976}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7199

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