What does this mr do?

Fix adding users to a course (e.g.: add teacher to course)

Actions taken to fix bug

Fix autowired mess of user/userservice that was used for checking for valid netIDs

Does this MR meet the acceptance criteria?

I have added a changelog entry to reflect the significant changes I made and the bug I fixed.
A test was created to test the bug.
I have updated the documentation accordingly.
I adhere to the style guide.