Allow auth server to fallback to password auth.

The authorisation server can now reply with 204 (no content)
in order to request that the client should continue with password
authentication.

static/protocol.js

@@ -477,7 +477,12 @@ ServerConnection.prototype.join = async function(group, username, credentials, d
                 throw new Error(
                     `The authorisation server said: ${r.status} ${r.statusText}`,
                 );
-            m.token = await r.text();
+            let data = await r.text();
+            if(!data)
+                // empty data, continue with password auth
+                m.password = credentials.password;
+            else
+                m.token = data;
             break;
         default:
             throw new Error(`Unknown credentials type ${credentials.type}`);