Resolve "Have the teacher/manager select which TA can handle which assignment"

What does this mr do?

Add the option for teachers to assign a specific assignment to specific TA's. Currently, if the list is empty all TA's are allowed to handle requests with this assignment. If there are 1 or more TA's selected for this assignment, the other TA's will not be able to handle these assignments.

Screenshots

before

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.