Removed possibility to create/edit empty exams/document/category

Closes #301 (closed) Closes #302 (closed)

Adds checks in frontend and backend to make sure no new documents/categories/exams can be created with blank or whitespaced names.

Additionally has migrations to add a temporary name to all currently empty documents/exams/categories.

I've now made document slugs a unique entry in the database. This will enforce that the duplicate document slug problem can't happen.