Resolve "Feedback on deleted request breaks assistant statistics page"

What does this mr do?

See #707 (closed) for details.

Actions taken to fix bug

Basically 404 since deleted request could not be found, when scanning through all associated feedbacks. Basically we now only consider feedbacks on the requests that were provided to the method.

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.
• I adhere to the style guide.

Closes #707 (closed)