add created_by to notifications

this is so one-off notifications can be tied to a user
(jobs have a created_by, and api notifications don't make sense
 to have one)

app/models.py

@@ -800,6 +800,9 @@ class Notification(db.Model):
     phone_prefix = db.Column(db.String, nullable=True)
     rate_multiplier = db.Column(db.Float(asdecimal=False), nullable=True)
 
+    created_by = db.relationship('User')
+    created_by_id = db.Column(UUID(as_uuid=True), db.ForeignKey('users.id'), nullable=True)
+
     @hybrid_property
     def status(self):
         return self._status_enum