MetaMTA - add support for "Default Author"

Summary: Ref T5952. This adds support for a "default author" and deploys it on Maniphest.

Test Plan: used augmented (by this diff) bin/mail receive-test to test creation via an application email with a default author configured and no author specified. a task was created with the author as the default author i configured.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T5952

Differential Revision: https://secure.phabricator.com/D11446