Backport common mixin and generics improvements

* Rename get_object_list to get_source_queryset.
* Add uniqueness validation to SingleObjectCreateView.
* Remove MultipleInstanceActionMixin.
* Backport MultipleObjectMixin improvements.
* Remove ObjectListPermissionFilterMixin.
* Add and improve tests.

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

mayan/apps/task_manager/views.py

@@ -16,7 +16,7 @@ class QueueListView(SingleObjectListView):
     }
     view_permission = permission_task_view
 
-    def get_object_list(self):
+    def get_source_queryset(self):
         return CeleryQueue.all()
 
 
@@ -33,7 +33,7 @@ class QueueActiveTaskListView(SingleObjectListView):
     def get_object(self):
         return CeleryQueue.get(queue_name=self.kwargs['queue_name'])
 
-    def get_object_list(self):
+    def get_source_queryset(self):
         try:
             return self.get_task_list()
         except Exception as exception: