Make "Receive Test" mail form use MailReceivers

Summary: Currently this is fairly hard-coded. Instead, make it use available receivers. Ref T1205.

Test Plan: Used mail form to send mail to various objects (Dnn, Tnn, Cnn, etc.). Only some of these work right now because the receiver thing still hard-codes a bunch of junk.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1205

Differential Revision: https://secure.phabricator.com/D5944