Fix calls to API client which now takes fewer args

The Notify API client changed in version 4 to take two arguments, not
three (service ID was removed in favour of the combined API key).

This gets a bit gnarly because the API key has to be at least a certain
length so it can be substringed internally.

app/notify_client/invite_api_client.py

@@ -5,12 +5,14 @@ from app.notify_client.models import InvitedUser
 
 class InviteApiClient(NotifyAdminAPIClient):
     def __init__(self):
-        super().__init__("a", "b", "c")
+        super().__init__("a" * 73, "b")
 
     def init_app(self, app):
         self.base_url = app.config['API_HOST_NAME']
-        self.service_id = app.config['ADMIN_CLIENT_USER_NAME']
-        self.api_key = app.config['ADMIN_CLIENT_SECRET']
+        self.api_key = '{}-{}'.format(
+            app.config['ADMIN_CLIENT_USER_NAME'],
+            app.config['ADMIN_CLIENT_SECRET'],
+        )
 
     def create_invite(self, invite_from_id, service_id, email_address, permissions):
         data = {