Fix waiting time for coaches

[sarah1507]

Waiting time of coaches for getting accepted by student needs fixing.
Fix the bug that a coach is still waiting for a student to accept his or her invitation, eventhough student is already connected to another coach:
2 coaches are simultaneously looking at a student's profile. Now, one coach decides to start a conversation with the student AND the conversation takes place. In this moment the 2nd teacher who is looking at the profile should be notified that the student now started a conversation with another coach ("Der Sprachschüler telefoniert schon!")