Replace hard-coded Steam CDN URLs with Asset "Keys"

[matanlurey]

Based on an idea by @dicewrench, would close #342.

---

Right now we directly refer to assets by the (steam) CDN URL:

templateInfo.speed3 = "http://cdn.steam.com/some/random/1234/url"

This is all fine and dandy, but if that URL is ever deleted (it happens!), the mod is effectively broken until it's re-published.

What if instead, we published an assets.json file:

{
  "speed3": "http://cdn.steam.com/some/random/1234/url"
}

And we referred to stuff indirectly, i.e.

_G.assets = {}

function getAsset(name)
  return _G._assets[name]
end

templateInfo.speed3 = getAsset("speed3")

This would be quite simple (programming wise, at least), and we could add tests to make sure that new URLs are never (directly) added to the mod. One big downside is that getAsset could never be called before the JSON file is downloaded, which means we'd have to be very careful with onLoad() code:

function onLoad()
  spawnModel({
    -- Error: assets.json has not been downloaded yet.
    mesh = getAsset("oneMeshyBoi")
  })
end