Adapt to HTML Modules

[tomalec]

As HTML Imports are dying there is a new proposal for a spec being discussed
WICG/webcomponents#645

Hopefully, we will be able to adapt our solution to the new spec once it is settled.

[tomalec]

Some inspiration https://github.com/matthewp/views-the-hard-way for the step before.

[tomalec]

Ideas for Service Worker as polyfill of HTML/CSS/JSON modules https://gist.github.com/developit/689aa4415bd688f3fce923cb8ae9abe7