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