Web client performance

Some different ideas for smoothing out the web client:

- The `.wasm` binary is around 50MB, which makes initial startup time quite sad. Gzipped is around 20MB. I can't find any option to enable this in `wasm-pack`, because I think people generally just configure their web servers to do compression on-the-fly if the HTTP client sends `Accept-Encoding` headers. S3 doesn't support this, but I think it's possible to upload a `.wasm.gz`, point to it from the `.html`, and set the `Content-Encoding` header properly. Just need to test this and make sure browsers understand it.
- Keep the `Scenario` for the current map in memory after loading it from a file. Otherwise, everytime the player resets to midnight in many gameplay modes, they have to download the file again!
- In `wasm_loader::FileLoader`, display a progress bar with bytes downloaded / total length. I found `set_onprogresss` for `XmlHttpRequest`, but actually, we're currently using `web_sys::Request`, which is different. Can we switch?