Fix task for sending of news digests blowing up on unauthorized users

* Trying to send a digest to a user without admin rights is raising
	a ActionForbiddenError
* This was causing the batch-sending of digests to blow up with
	unauthorized users
* The exception is now rescued and a helpful message is printed,
	the batch can continue with the other users
Turn user names to links in admin
Make user subribed to news digest by default