Make "new task" and "new conpherence" not-so-awful

Summary: Ref T1205. Moves the handling logic for these email types to reply handlers.

Test Plan: Used test form to send conpherence and maniphest mail.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1205

Differential Revision: https://secure.phabricator.com/D5945