Handle archive links

This is a first pass at dealing with clicks on links in archive playbacks. It just shows an interstitial that tells you you're leaving Perma, then you can click the original link.

This doesn't yet offer a link to IA, which is going to be a hassle because PyWB doesn't actually know the date of the archive it's trying to play back. I think this is way better than the current situation though -- vote we roll this out next week and iterate from there.