Builtin Prosody: use Peertube domain instead of localhost.

server/lib/prosody/config/domain.ts

@@ -0,0 +1,12 @@
+async function getProsodyDomain (options: RegisterServerOptions): Promise<string> {
+  const url = options.peertubeHelpers.config.getWebserverUrl()
+  const matches = url.match(/^https?:\/\/([^:/]*)(:\d+)?(\/|$)/)
+  if (!matches) {
+    throw new Error(`Cant get a domain name from url '${url}'`)
+  }
+  return matches[1]
+}
+
+export {
+  getProsodyDomain
+}