Display course statistics and improve the course deletion dialog

[krusche]

When deleting a course, the current dialog is very simple and looks identical for all kinds of courses

This can lead to confusion when deleting multiple courses and - in the worst case - can lead to deleting the wrong course.

It would be better, if the user can see certain information before confirming the deletion of the course:

• Number of exercises (ideally per type)
• Number of repositories (i.e. participations for programming exercises)
• Number of builds
• Number of exams (ideally with number of involved exercises and users)
• Number of lectures
• Number of communication posts and answer posts
• Number of competencies
• Number of tutorials
• Number of enrolled students, tutors, editors and instructors
• ...
• Whether this is a test course or not.

Ideally those numbers are displayed in a new statistics component on the course details page (e.g. small table on the top right) anyway and are downloaded within one request. Then, the corresponding DTO can be passed into the delete dialog and would not need to be fetched when opening the delete dialog.