Add config option to disallow automatic study group creation

[MrSerth]

In order to make copying credentials across courses on Xikolo more difficult (and prevent nice groups in CodeOcean for our research), we should add an option (!) to disallow automatic study group creation. If no study group is available for the current course_context it should not be created and learners, as well as teachers, should not get access to the exercise or gain any permissions. Instead, an error message should be shown. Only an administrator should be able to manually proceed and confirm the creation of the new study group.