QTI: add chat to run-time for interaction with supervisor

For open-book exams where the examinees are at home it is necessary that the examinee can contact the supervisor in case there is a question or a technical problem. The idea is to leverage the existing chat feature to implement such a functionality (at least conceptually if technically there is nothing to leverage):

• In the test course element configuration enable / disable the chat with supervisor feature
• For Supervisor add a chat link for each examinees in the assessment tool table to contact the examinee
• For Examinees add a chat link button in the header of the test runtime to contact the supervisor
• When a chat message is available, open the chat window right away
• Make sure the chat for this works also in assessment mode
• Optional: Save the chat log for each examinee to disk so it is part of the archive