Part ofcoder/internal#19

Users should be empowered to control which notifications they do not wish to receive.

Added two new routes:

• GET /users/{users}/notifications/preferences
  • Retrieves notification preferences for a given user
• PUT /users/{users}/notifications/preferences
  • Updates notification preferences for a given user

When a user has disabled a notification template in their preferences, we need to ensure that:

1. Messages currently in the queue which use that (now disabled) notification template are inhibited from sending
2. Any new messages which are enqueued after the preference is set will fail when being inserted into the database, and the enqueuer will return anotifications.ErrCannotEnqueueDisabledNotification error

The database logic was implemented in#14100, see theinhibit_enqueue_if_disabled trigger.