Fix mentorgroup uploading

What does this mr do?

• mentor groups were displayed with all details instead of a simple name due to Lombok
• every student gets a unique mentorgroup instead of everyone

Actions taken to fix bug

• override toString
• Add check for existing mentorgroup before creating new ones

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.