Fetch classlist to add Lecturer and User to DocumentCategory

Sadly I could not get all the lecturers that are associated with the lectures #133  from studien.hsr.ch.
This approach is a bit different, it parses the class lists of the current semester and aggregates all the lecturers from the different exercises and lectures of a module.

When parsing the class list we can also find out, which of our users are registered in which module. This provides data for future features like showing only the modules that the user is registered to in the current semester. That's why I added a Many-2-Many relationship between the users and modules.

We could do even more things with the data of the class list, like pulling out the first_name and last_name or the student photo (but I'm not sure if I would want that as a student :grin: ).

Interesting fact I found out while I ran this:
Of the current semester 538 users are not registered (and I even know which ones :grin:) to the studentenportal while 725 users are.

Edit: Uups just saw all my PEP violations, will fix that first