Allow any user to upload exam PDFs

Currently only category admins ca upload exam PDFs, after which they need to be cut and made public. This limits the amount that a user can contribute to the growth of the File Collection. Instead of a model where users need to ask/request exams to be added, it should rather be one where they can upload freely but need approval before it becomes public.

This MR implements the functionality for all users to be able to upload exams to any category. Approval (in the form of cutting and making it public) still requires a category admin. The Mod Queue displays all exams that need processing.

To supplement this change, the frontend category page also has a new button to upload exams for that category without having to go to /uploadpdf.