switch url to id in invitation process

pb/messages.proto

@@ -19,9 +19,9 @@ message PackedServerMessage {
 message Invitation {
     bytes payload = 1;
     int32 timeout = 2;
-    int32 urllen = 3;
+    int32 idlen = 3;
     string password = 4;
-    string url = 5;
+    string id = 5;
     int64 expiry = 6;
     int32 step = 7;
 }