Slim down the /organisations response

At the moment this response returns a list of service IDs for hundreds
of organisations.

The admin app doesn’t use this information, but having to wait for it to
be serialized and sent across the network slows it down all the same.

app/organisation/rest.py

@@ -45,7 +45,7 @@ def handle_integrity_error(exc):
 @organisation_blueprint.route('', methods=['GET'])
 def get_organisations():
     organisations = [
-        org.serialize() for org in dao_get_organisations()
+        org.serialize_for_list() for org in dao_get_organisations()
     ]
 
     return jsonify(organisations)