Create a separate route for exam edit page

The exam page /exam/:filename can be in two vastly different states: the main PDF page, and the metadata edit page.

Having both of them be on the same URL is bad UX for admins, so I have separated the edit page into a /exam/:filename/edit subpage using a local Route component. I've also made it so it will redirect to the main PDF page if you're unauthorised.

All the logic is still confirmed to be working.

This is very similar to !534 (merged) but for exam pages.