Gatsby core won't have SW precaching built-in in the next release so
this is necessary for a fast site. And even if a browser is precaching
bundles, pulling bundles from the SW and evaling them still adds latency
to page transitions. Doing this work ahead of time in spare moments
means that page transitions will be always be fast.

The webpack offline-plugin is great but it doesn't work with content not
handled by Webpack as Gatsby is increasingly moving towards.

Also by handling this in plugins instead of core, it'll offer users more
choices about how to handle "offline" than trying to meet all needs in
core.

Overly expensive for pages with lots of links.

This adds a basic setup for handling loading a website with a service
worker generated by sw-precache. It uses the AppShell technique to load
first a simple HTML shell of the website (which would generally show the
header for most sites) which then loads the Javascript for the actual
page and renders that.