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.
Reviewers: chad, btrahan
Reviewed By: btrahan
Subscribers: epriestley
Differential Revision: https://secure.phabricator.com/D10240