Cache downloadable game binaries

[ufna]

It's really not cool to download assets each time you open the page. Game assets and binaries should be cached with defined versions and reused.

Possible helpful links:

• https://answers.unrealengine.com/questions/791739/emscripten-html5-with-multiple-pak-possible.html
• https://answers.unrealengine.com/questions/416588/html-build-packaging-broken-for-shipping-builds.html
• https://answers.unrealengine.com/questions/144136/how-to-reduce-html5-package-size.html
• https://github.com/emrahgunduz/unrealEngine-html-pak-downloader