Maniphest / MetaMTA config options

Summary: Move the Maniphest-related mta options into config.

Test Plan: Looked at options and edited a couple. Looked at setup warnings to make sure the relevant setup warnings were no longer raised.

Reviewers: codeblock, btrahan

Reviewed By: codeblock

CC: aran

Maniphest Tasks: T2255

Differential Revision: https://secure.phabricator.com/D4465