* removes 'revocation' folder if RevocationDocument is removed from an intervention and removes the whole intervention document folder as well, if no other documents for this intervention exist
* adds further documentation
* refactors getting of related documents into Intervention model
* increases max_size for FileFields --> triggers when the file name will be automatically changed during upload
* adds Http404 Exception in case document file does not exist anymore on the hard drive
* refactors documents and file upload to be distributed into different subfolders, depending on the type of document (InterventionDocument, RevocationDocument, ...)
* refactors Document model into AbstractDocument
* subclasses RevocationDocument, InterventionDocument, COmpensationDocument, EmaDocument and EcoAccountDocument from AbstractDocument to provide proper functionality for each
* adds new specialized routes for each new document type (opening, removing)
* drops generic get and remove routes for documents
