Bug 1030996 - Add in tool notifications

- When a Task becomes unavailable (e.g., by expiring or being marked as a draft),
  find all attempts for the Task and mark them as requiring notification
- Using middleware, if an attempt for the current user is marked as `requires_notification`, display a modal message
  informing them that their open task was closed
Merge pull request #155 from bobsilverberg/notifications

Bug 1030996 - Add in tool notifications