Remove hard coded strings for the notification popup

[crankycoder]

There's well defined i10n strings that are available for the notification popup available:

https://dxr.mozilla.org/mozilla-central/source/browser/modules/SitePermissions.jsm#110

  /* Returns the localized label for the given permission state, to be used in
   * a UI for managing permissions.
   */
  getStateLabel: function (aPermissionID, aState) {
    switch (aState) {
      case this.UNKNOWN:
        return gStringBundle.GetStringFromName("alwaysAsk");
      case this.ALLOW:
        return gStringBundle.GetStringFromName("allow");
      case this.SESSION:
        return gStringBundle.GetStringFromName("allowForSession");
      case this.BLOCK:
        return gStringBundle.GetStringFromName("block");
      default:
        throw new Error("unknown permission state");
    }
  }
};

Copy/pasta that into the addon