Add option to allow inviting other users in portal rooms (#77)

This doesn't mean the invited users get bridged, but for some use cases that doesn't matter

example-config.yaml

@@ -101,6 +101,10 @@ bridge:
     # but causes room avatar/name bugs.
     private_chat_portal_meta: false
 
+    # Allow invite permission for user. User can invite any bots to room with whatsapp 
+    # users (private chat and groups)
+    allow_user_invite: false
+
     # The prefix for commands. Only required in non-management rooms.
     command_prefix: "!wa"