Add last-minute check to ensure master views allows deletion

Generally for the 'delete' route to even exist the master view *class*
must still allow deletions.  But once a request is involved and we have
a view *instance* then we can disable deletions if we like.

tailbone/views/master.py

@@ -166,6 +166,9 @@ class MasterView(View):
         """
         View for deleting an existing model record.
         """
+        if not self.deletable:
+            raise httpexceptions.HTTPForbidden()
+
         self.deleting = True
         instance = self.get_instance()
         instance_title = self.get_instance_title(instance)