avoid extraneous queries to Notification

just use the cached .all, even for associations

test plan:
 * go to the notification preferences page as a user
 * in console
   * np = NotificationPolicy.first; np.notification
   * the first time, it should query notification_policy,
     and *all* notifications
   * do it again, and this time it should not query
     notifications, but should still return an object

Change-Id: I3b83ec9ce6f6f6b4d542e83d2b8653b4e221afd4
Reviewed-on: https://gerrit.instructure.com/40460
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>