Refactored documents app
/label technical change
Closes #268 (closed), #519 (closed)
Short description
Refactored the documents app. I added a base Document model, with AnnualDocument, AssociationDocument, MiscellaneousDocument and Minutes as subclasses. This removes a lot of duplication, e.g. there now is only one function for downloading a document. This also makes it possible to link events and documents in the future, or to do other cool stuff. I also made everything bilingual.