Add a setting to disable all notification email

Summary: Ref T5861. Adds an option to opt out of all notification email. We'll still send you password resets, email verifications, etc.

Test Plan:

• Added unit tests.
• With preference set to different things, tried to send myself mail. Mail respected preferences.
• Sent password reset email, which got through the preference.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: rush898, epriestley

Maniphest Tasks: T5861

Differential Revision: https://secure.phabricator.com/D10237