Demo Site: show progress while downloading gputop-web.js

[rib]

gputop-web.js is currently > 1MB of JavaScript compiled via Emscripten and until that gets downloaded the demo can't show the lists of available metrics, but it doesn't give any indication that it's being downloaded.

An alternative to showing a progress, could be to see if we can instead download a compressed version of the file and find a small decompression client side decompression library.