Resolve "toString method of FirstYearMentorGroup causes StackOverflow"

What does this mr do?

It solves a bug where calling firstYearStudent.toString or firstYearMentorGroup.toString caused a stackOverflowError.

Actions taken to fix bug

Add a toString method to FirstYearStudent such that it doesn't create an infinite recursive call.

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.

Closes #326 (closed)