Don't serialize broadcast_messages in TemplateHistorySchema

In 0282a76 we excluded serializing `broadcast_messages` on some schemas.
This also excludes it from the TemplateHistory schema.

app/schemas.py

@@ -407,6 +407,7 @@ class TemplateHistorySchema(BaseSchema):
 
     class Meta(BaseSchema.Meta):
         model = models.TemplateHistory
+        exclude = ('broadcast_messages',)
 
 
 class ApiKeySchema(BaseSchema):