Move mail "Receive Test" from web UI to CLI

Summary:
Ref T3306. Moves this from the web to the CLI, which is a tiny bit clunkier but way better as far as policies go and more repeatable for development.

See discussion in D6413.

Test Plan: Ran `bin/mail receive-test`, verified mail was received. Used and abused various options.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3306

Differential Revision: https://secure.phabricator.com/D6417