Allow users to set notifications to "Email", "Notification", or "Ignore"

Summary:
Ref T5861. Ref T5769. If users don't care at all about something, allow them to ignore it.

We have some higher-volume notifications either built now (column changes) or coming (mentions) which users might reasonably want to ignore completely.

Test Plan:
Ignored some notifications, then took appropraite actions. Saw my user culled from the notification subscriber list.

{F189531}

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5769, T5861

Differential Revision: https://secure.phabricator.com/D10240