Improve Cloudfront config

Broken: http://play.abstreet.org/0.3.43/santa.html
Working: http://abstreet.s3-website.us-east-2.amazonaws.com/0.3.43/santa.html

```
Started loading WASM [santa.bundle.js:83:21](http://play.abstreet.org/0.3.43/santa/santa.bundle.js)
19:26:51.623 state change: loading -> error [santa.bundle.js:166:17](http://play.abstreet.org/0.3.43/santa/santa.bundle.js)
19:26:51.638 Assertion failed: not yet loaded [santa.bundle.js:129:21](http://play.abstreet.org/0.3.43/santa/santa.bundle.js)
19:26:51.639 state change: error -> starting [santa.bundle.js:166:17](http://play.abstreet.org/0.3.43/santa/santa.bundle.js)
19:26:51.639 running app with assetsBaseURL: ./data, assetsAreGzipped: true [santa.bundle.js:132:25](http://play.abstreet.org/0.3.43/santa/santa.bundle.js)
19:26:51.640 state change: starting -> error [santa.bundle.js:166:17](http://play.abstreet.org/0.3.43/santa/santa.bundle.js)
19:26:51.660 Uncaught (in promise) TypeError: wasm is undefined
```

The network tab doesn't even show a .wasm file being loaded:

This appears to have quietly broken a week or two ago, despite there being no recent changes anywhere (S3 or Cloudfront activity generally). I invalidated the entire Cloudfront cache to be safe, but no effect.

IIUC, it looks like Cloudfront stopped returning `Content-Length` headers? We try to grab that before streaming data back:
https://github.com/a-b-street/abstreet/blob/4314a43e777f26c040a20bd6022cb57be00b3389/web/src/widgetry.ts#L121

@michaelkirk if you happen to have any ideas what could've broken, I'd welcome some help! But no pressure, I'm comfortable enough in TS to figure this out now. :)