[Mentors] New alerts if mentor request is cancelled

If a student cancels their mentor request while you’re working on giving the first feedback, we’ll now alert you with a modal that will pop up. If you’ve started writing, we’ll give you the option of closing the modal in case you want to copy the text you’ve written. Otherwise you can click on the primary button to head back to the queue.

Hopefully this removes some of the frustration when this happens and means you waste as little time as possible.