Add pk3 file caching to emscripten port

[zturtleman]

An issue for deploying the emscripten port for standalone games is that the pk3 files are not cached by the browser. They're re-downloaded every time the page is loaded. This causes excessive server bandwidth usage.

It's possible to store the files in the browser's IndexedDB. There are emscripten C functions for this. Though currently the web client fetches the files in Javascript. It probably also need to check for updated files on the server.