Resolve "Support for the notion of groups"

What does this mr do?

Adds a warning message that labs will break if no groups are imported. When no groups are imported and trying to create a lab it will give a warning that groups need to be imported. It will also restrict access to the lab creation page if a course has groups and there are no mentor groups available. This such that there is no way a course can create a lab without any students in it.

Also solves a small issue where a teacher accidentally checks the "uses groups" checkbox and now it can be disabled once.

Screenshots

after

Does this MR meet the acceptance criteria?

• I have added a changelog entry to reflect the significant changes I made.
• Tests were created to test the feature.
• I have updated the documentation accordingly.
• I adhere to the style guide.

Closes #100 (closed) Closes #117 (closed)