Mailgun receive support
As you've suggested, I took the SendGrid code and massaged it until it played nice with Mailgun.
btw - unless I'm missing something, it appears that the SendGrid receiver lets you spoof emails (it performs no validation on the data received).
Test Plan: Opened a task with Mailgun. Felt great.
Reviewers: epriestley, Blessed Reviewers
Reviewed By: epriestley
CC: Korvin, epriestley, aran
Maniphest Tasks: T4326
Differential Revision: https://secure.phabricator.com/D7989