Add endpoints to set user notification preference

Two endpoints are intended to be used by AJAX calls from the web UI with an
authenticated user. When enabled, the value of the preference is an encryption
of the username, to be used as a token for one-click unsubscribe links. The
third endpoint is the target of unsubscribe links, which displays a page with
an appropriate message to the user.
Add acceptance test for one-click unsubscribe page
Merge pull request #459 from edx/gprice/notification-setting-endpoints

Add endpoints to set user notification preference