Previously, when you clicked the Add button, the page wouldn't change.

It looked like it wasn't doing anything, as noted by @rcarmo
(ArchiveBox#658 (comment))

I didn't track it down the exact reason why. It may be that Safari
didn't like the two opening <h3>s, but I was able to find a bunch of
people complaining about Safari being very finicky with innerHTML
and actually repainting the page, enough that I decided to try just
extending the block hide/show behavior already done with the
delay-warning, and it works for me now in both Chrome and Safari.

For ArchiveBox#658.