Teacher participations are saved in quiz-student relation table

[Laczse]

Teacher participations are saved in quiz-student relation table as there are no other place to store them.
This creates problems for editing and deleting questions, as well as being against the desig of the database.
There is currently a solution implemented for editing and deleting questions, however it is not very elegant.

To fix this issue the following should be done:

1. Update the database to have a quiz-teacher relation table and a teacher-answer relation table (This should be done in install.xml and upgrade.php)
2. Create the required methods to insert and retrieve data from/to the new table
3. Remove the code in view.php that deletes all quiz-student relations by id and all student-answer relations by id
4. Update the code in submit_quiz.php to differentiate between students and teachers. Saving their participations and answer relations in the correct table
5. Update code that retrieves participations and answer-student relations to differentiate between teachers and students. (This is partly in results.php and results_page.php, but other places may be relevant)
6. Manually test to see if you can take a livequiz as a teacher and then edit the questions
7. Run behat tests to ensure everything still works