Improve response times

[Changaco]

• The home page is taking around 1 second to generate in production, that's too slow, and it's 5 times what I'm getting locally.
• We also have a problem with the asset compiler, I was saying yesterday in First time setup fixes gratipay/gratipay.com#3911 (comment) that it's elegant, but apparently it still has a few bugs too. :D Sentry issue: https://sentry.changaco.oy.lc/sentry/liberapaycom/issues/2/

[Changaco]

It looks like the two issues were linked, because response times have dropped after the deployment of #117. Due to a bad env var we only had one worker, and my guess is that serving the non-cached files was blocking the responses to the other requests.