Homepage startup speed improvements

Profile: https://share.firefox.dev/3nxO0rj

Main take aways:

* before-load.js takes a lot of time => we could at least inline it inside index.html. But we can also look at whether it could be changed: the redirection part could possibly be replaced with [Clear-Site-Data](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Clear-Site-Data) (if netlify allows it). Maybe it's also fine that this script is ran with "async" attribute, we may not need it to be synchronous.
* [extracting CSS](https://github.com/firefox-devtools/profiler/issues/1233) could be useful
* [preloading](https://developer.mozilla.org/en-US/docs/Web/HTML/Link_types/preload) app.ftl for english could be good

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-502)