Calculate timemap when publishing web archives

[jcoyne]

Currently the viewer is dynamically looking up the timemap url based on the "Archived website" url in the MODS.

https://github.com/sul-dlss/sul-embed/blob/921130b790b8b7babd41c2e004535e3b12028303/app/viewers/embed/viewer/was_seed.rb#L32-L40

This process could error (because networks are not 100% reliable), and present an error to the user. Instead we should do this process at publish time, where it can retry (since publish is a robot) if there is a failure.