Make "subscribed" filter in Differential accept any mailable

Summary:
In the Differential revision list views:

• Allow you to filter by mailables (notably, mailing lists).
• Allow you to filter by user (including disabled users).

Test Plan: Filtered by a mailing list.

Reviewers: btrahan, nh

Reviewed By: nh

CC: aran, epriestley

Maniphest Tasks: T1031, T1034

Differential Revision: https://secure.phabricator.com/D1994