Add support for 'department' field in purchases / batches

Also fix logic for deleting a purchase (delete its batches first)

tailbone/views/master.py

@@ -526,7 +526,12 @@ class MasterView(View):
         Return a "humanized" (and plural) version of the model name, for
         display in templates.
         """
-        return getattr(cls, 'model_title_plural', '{0}s'.format(cls.get_model_title()))
+        if hasattr(cls, 'model_title_plural'):
+            return cls.model_title_plural
+        try:
+            return cls.get_model_class().get_model_title_plural()
+        except (NotImplementedError, AttributeError):
+            return '{}s'.format(cls.get_model_title())
 
     @classmethod
     def get_route_prefix(cls):