Slow mode WIP (#192):

* default delay is configurable in channel's chat rooms options.

shared/lib/types.ts

@@ -74,6 +74,9 @@ interface ChannelConfigurationOptions {
     }>
     // TODO: bannedJIDs: string[]
   }
+  slowMode: {
+    defaultDelay: number
+  }
 }
 
 interface ChannelConfiguration {