Slow mode WIP (#192):

* new option in room configuration to set the slow mode delay (new prosody module mod_muc_slow_mode).

server/lib/prosody/config/content.ts

@@ -205,6 +205,8 @@ class ProsodyConfigContent {
     this.muc.set('muc_room_default_public_jids', false)
     this.muc.set('muc_room_default_change_subject', false)
     this.muc.set('muc_room_default_history_length', 20)
+
+    this.muc.add('modules_enabled', 'muc_slow_mode')
   }
 
   useAnonymous (autoBanIP: boolean): void {