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/document_comments/views.py

@@ -118,7 +118,7 @@ class DocumentCommentListView(SingleObjectListView):
             'title': _('Comments for document: %s') % self.get_document(),
         }
 
-    def get_object_list(self):
+    def get_source_queryset(self):
         AccessControlList.objects.check_access(
             obj=self.get_document(), permissions=(permission_comment_view,),
             user=self.request.user