message: Let users know if mentions have been disabled
diff: @@ -737,12 +737,15 @@ MatrixHandler.prototype._onAdminMessage = Promise.coroutine(function*(req, event
  args[1].toLowerCase() === "true";
  yield this.ircBridge.getStore().storeUserFeatures(event.user_id, features);
  + let note = "";
  if (featureFlag === "mentions") {
  // We should invalidate caching for this user's channels.
  - this.ircBridge.ircHandler.invalidateCachingForUserId(event.user_id);
  + if(!this.ircBridge.ircHandler.invalidateCachingForUserId(event.user_id)){
  + note = " This bridge has disabled mentions, so this flag will do nothing.";
  + }
  }
  let notice = new MatrixAction("notice",
  - `Set ${featureFlag} to ${features[featureFlag]}`
  + `Set ${featureFlag} to ${features[featureFlag]}.${note}`
  );
  yield this.ircBridge.sendMatrixAction(adminRoom, botUser, notice, req);
  }
  