add a few more email preferences for differential and maniphest

Summary: this makes notifications work better for folks who choose to handle things in Phabricator and not over email

Test Plan: had my test account and "real" account battle each other on a few tasks and divs. Noted that I received emails appropos to the respective settings.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1977

Differential Revision: https://secure.phabricator.com/D3856