Move add_to_class functions to their own module

* The new module is called methods.py and found on each app.
* Add keyword arguments to add_to_class instances.
* Remove catch all exception handling for the check in and
  check out views.
* Improve checkouts tests code reducing redundant code.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/cabinets/api_views.py

@@ -39,7 +39,7 @@ class APIDocumentCabinetListView(generics.ListAPIView):
             obj=document
         )
 
-        queryset = document.document_cabinets().all()
+        queryset = document.get_cabinets()
         return queryset