Add broadcast channel to service schema

This will show which channel is configured, if any, for a service. It
mimics what we are doing for the `allowed_broadcast_provider`.

app/schemas.py

@@ -236,10 +236,14 @@ class ServiceSchema(BaseSchema, UUIDsAsStringsMixin):
     override_flag = False
     go_live_at = field_for(models.Service, 'go_live_at', format=DATETIME_FORMAT_NO_TIMEZONE)
     allowed_broadcast_provider = fields.Method(dump_only=True, serialize='_get_allowed_broadcast_provider')
+    broadcast_channel = fields.Method(dump_only=True, serialize='_get_broadcast_channel')
 
     def _get_allowed_broadcast_provider(self, service):
         return service.allowed_broadcast_provider
 
+    def _get_broadcast_channel(self, service):
+        return service.broadcast_channel
+
     def get_letter_logo_filename(self, service):
         return service.letter_branding and service.letter_branding.filename
 

tests/app/service/test_rest.py

@@ -243,6 +243,7 @@ def test_get_service_by_id(admin_request, sample_service):
     assert json_resp['data']['email_branding'] is None
     assert json_resp['data']['prefix_sms'] is True
     assert json_resp['data']['allowed_broadcast_provider'] is None
+    assert json_resp['data']['broadcast_channel'] is None
 
     assert set(json_resp['data'].keys()) == {
         'active',
@@ -250,6 +251,7 @@ def test_get_service_by_id(admin_request, sample_service):
         'billing_contact_email_addresses',
         'billing_contact_names',
         'billing_reference',
+        'broadcast_channel',
         'consent_to_research',
         'contact_link',
         'count_as_live',