Add 'stubbed' column to broadcast_message table

This is a boolean column. It will be set to True for broadcasts
created from training broadcast accounts.

This will help us debug, for example by excluding all the stubbed
broadcasts when we have some trouble with real broadcasts.

app/models.py

@@ -2269,6 +2269,8 @@ class BroadcastMessage(db.Model):
 
     reference = db.Column(db.String(255), nullable=True)
 
+    stubbed = db.Column(db.Boolean, nullable=True)
+
     CheckConstraint("created_by_id is not null or api_key_id is not null")
 
     @property