Homepage startup speed improvements

[julienw]

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 (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 could be useful
• preloading app.ftl for english could be good

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