FIX: If there is no `last_posted_at` don't raise an exception
FIX: Don't raise an exception when `last_posted_at` is nil
BUGFIX: buttons have a different size when clicked on FF (cc @awesomerobot)

cf. https://meta.discourse.org/t/browse-buttons-in-the-edit-history-have-a-different-size-when-clicked/16698