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/document_states/apps.py

@@ -83,7 +83,7 @@ class DocumentStatesApp(MayanAppConfig):
         )
 
         Document.add_to_class(
-            'workflow', DocumentStateHelper.constructor
+            name='workflow', value=DocumentStateHelper.constructor
         )
 
         ErrorLogEntry.objects.register(model=WorkflowStateAction)