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/nick.ts

@@ -0,0 +1,9 @@
+function randomNick (base: string): string {
+  // using a 6 digit random number to generate a nickname with low colision risk
+  const n = 100000 + Math.floor(Math.random() * 900000)
+  return base + ' ' + n.toString()
+}
+
+export {
+  randomNick
+}