Call to _send_queue_mail only when registration is not on queue and before cancelling it

Closes #448 (closed) and #447 (closed)

Previous behaviour

Steps to reproduce:

1. Create event with max participants
2. Add event and multiple registrations exceeding max participants
3. Cancel registration on the queue in the frontend
4. Mail sent to someone
5. Cancel registration not on the queue in the frontend
6. Mail sent to the wrong person

New behaviour

Steps to validate that it works:

1. Create event with max participants
2. Add event and multiple registrations exceeding max participants
3. Cancel registration on the queue in the frontend
4. No mail sent
5. Cancel registration not on the queue in the frontend
6. Mail sent to the right person