Improved handling of course enrollment

[Zitrone44]

Is your feature request related to a problem? Please describe.
Currently, students need to click 'enroll' in a course for their submissions to be visible to the docent of the course. This leads to problems when students ignore the large banner asking them to enroll and finish their tasks, but do not get them credited because they did not enroll.

Describe the solution you'd like
There are multiple possible solutions:

1. Auto-enroll students on first task submission.
2. Block task submission of non-"Exercise" tasks until they enroll.