Forum Sharing Integration Tests
This adds integration tests for these cases:
- normal invitation session where invitee accepts
- normal invitation session where invitee declines
- session where invitee leaves again after she joined
- session where sharer leaves after invitee has joined
- session where sharer leaves before invitee can respond
- sharer reuses the session ID of the previous session
- after accepting the invitation, the invitee invites the sharer to the same forum
- after session contacts delete each other and session states get cleaned up
Closes #339 (closed)