Add CSS class to watchlist pending notice

- Updated FlaggedRevsUIHooks.php to add a CSS class to the warning box
  for watchlist pending edits notice.
- Added 'mw-fr-watchlist-pending-notice' class to the Html::warningBox
  method to improve styling and identification.

Bug: T191156
Change-Id: I1854292820173ff21a4c3f7af03a4827184720fa
1 file changed