Resolve "Average processing time per TA"

What does this mr do?

• Refactors a bunch of code related to assistant stats.]
• Implements assistant stats to be easily viewable in the feedback page.

Screenshots

Student View

• Remains the same, since we don't display these statistics to the student.

Teacher View

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 #594 (closed)