MetaMTA - add (basic) application emails and deploy to Maniphest

Summary: Ref T5952, T3404. This lays the basic plumbing for how this will work, all the way to deploying on Maniphest. Aside from what is mentioned on T5952, I think page(s) on editing application emails could use a little more helpful text about what's going on, similar to how the config page that's getting deprecated works.

Test Plan: ran migration and noted my create email address migrated successfully. used bin/mail to make a task. added another email and used bin/mail to make a task. deleted an email. edited an email. invoked various error states and they all looked good.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T3404, T5952

Differential Revision: https://secure.phabricator.com/D11418