Don't eagerly load all JS assets

[mary-ext]

Realized that Bluesky is currently eagerly loading all languages and some code that was meant to be lazily-loaded, then noticed that Expo? Webpack? generates an asset-manifest.json which contains the actual entrypoint scripts to put on the HTML file.

Anyway, that saved time from having to figure out plugins like webpack-assets-manifest which is what I was about to pull beforehand.

(top: this PR, bottom: main.bsky.dev)

This pull request shaves ~2 MB of JS assets and some precious processing time spent during initial page load. It's still not ideal, but better than where we were before.

[mary-ext]

#3925 pulls in react-image-crop which requires us to import a CSS file. Thankfully it seems that the manifest adds in CSS files into the entrypoint array. Additionally, that would mean we can also move styling off the <style> element in web/index.html and bskyweb/templates/base.html (and also minify them!). I'll do these in a separate pull request if interested.