Add DB table for service broadcast settings

This will allow us to store details of which channel a service should be
sending to.

See the comment about how all broadcast services can have a row in the
table but may not at the moment. This has been done for speed as it's
the quickest way to let us set up different services to send to
different channels for some needed testing with the mobile handset
providers in the coming week.

app/models.py

@@ -505,6 +505,7 @@ class Service(db.Model, Versioned):
         backref=db.backref('services', lazy='dynamic'))
 
     allowed_broadcast_provider = association_proxy('service_broadcast_provider_restriction', 'provider')
+    broadcast_channel = association_proxy('service_broadcast_settings', 'channel')
 
     @classmethod
     def from_json(cls, data):
@@ -2519,6 +2520,39 @@ class BroadcastProviderMessageNumber(db.Model):
     )
 
 
+class ServiceBroadcastSettings(db.Model):
+    """
+    For the moment, broadcasts services CAN have a row in this table which will configure which broadcast
+    channel they will send to. If they don't then we will assume they should send to the test channel.
+
+    There should only be one row per service in this table, and this is enforced by
+    the service_id being a primary key.
+
+    TODO: We should enforce that every broadcast service will have a row in this table. We will need to do
+    this when the admin turns a service into a broadcast service, it inserts a row into this table and adds
+    the service permission for broadcasts for the service. Once that is up and running, we then should write
+    a DB migration to create rows for all broadcast services that do not have one yet in this table.
+
+    TODO: Move functionality on the ServiceBroadcastProviderRestriction into this table and remove the
+    ServiceBroadcastProviderRestriction table
+    """
+    __tablename__ = "service_broadcast_settings"
+
+    service_id = db.Column(UUID(as_uuid=True), db.ForeignKey('services.id'), primary_key=True, nullable=False)
+    service = db.relationship(Service, backref=db.backref("service_broadcast_settings", uselist=False))
+    channel = db.Column(
+        db.String(255), db.ForeignKey('broadcast_channel_types.name'), nullable=False
+    )
+    created_at = db.Column(db.DateTime, nullable=False, default=datetime.datetime.utcnow)
+    updated_at = db.Column(db.DateTime, nullable=True, onupdate=datetime.datetime.utcnow)
+
+
+class BroadcastChannelTypes(db.Model):
+    __tablename__ = 'broadcast_channel_types'
+
+    name = db.Column(db.String(255), primary_key=True)
+
+
 class ServiceBroadcastProviderRestriction(db.Model):
     """
     Most services don't send broadcasts. Of those that do, most send to all broadcast providers.