R1:be1fbba69858

Implement a "!subscribe" mail command

Summary: Ref T7199. Implements "!subscribe".

Test Plan: Used `bin/mail receive-test` to apply the command to objects.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7199

Differential Revision: https://secure.phabricator.com/D12247