Bundle offline templates for each core theme

[westonruter]

To support a themed offline page when using a core theme without necessitating the creation of a child theme, the PWA plugin should bundle offline page templates for each core theme.

The method for bundling can be to use template_include filter to inject the plugin path. See https://gist.github.com/westonruter/900611f075374c5468ea492f23ad4810