R1:ea5462fb600b

MetaMTA - lay some ground work for having an application

Summary:
Ref T5791. This does a few bits there. Namely:

- Adds PHID column to PhabricatorMetaMTAMail
- Implements a PhabricatorMetaMTAMailPHIDType
- Script to backpopulate them.
- Makes PhabricatorMetaMTAMail implement PolicyInterface.
- View policy is NOONE and the author and recipients have automatic view capabilities
- No edit capability.
- Adds a PhabricatorMetaMTAMailQuery for PhabricatorMetaMTAMail.

Test Plan: ran `.