Chat Federation: using S2S if available.

* if both local and remote instance have external XMPP connections enabled, the user joins the remote room with his local account
* some code refactoring (builtin.ts)

Note: documentation and settings descriptions are to do.

Related to #112

conversejs/lib/utils.ts

@@ -0,0 +1,11 @@
+function inIframe (): boolean {
+  try {
+    return window.self !== window.top
+  } catch (e) {
+    return true
+  }
+}
+
+export {
+  inIframe
+}