Make Open Food Facts a stellar Progressive Web App (PWA)

[teolemon]

What

• Progressive Web App report: pwa-report.pdf

### Tasks
- [ ] Insert things we can act on here

from @hangy I currently don't see how we could fulfill Progressive web apps work offline. usefully. But here are some of the TODOs apart from the manifest:

• [ ] Has a registered Service Worker
• [ ] URL responds with a 200 when offline
• [ ] Site uses HTTP/2 for its own resources
• [ ] Site does not use that delay first paint
  • https://static.openfoodfacts.org/foundation/css/app.css (delayed first paint by 548ms)
  • https://static.openfoodfacts.org/foundation/foundation-icons/foundation-icons.css (delayed first paint by 267ms)
  • https://static.openfoodfacts.org/js/jquery-ui-1.11.4/jquery-ui.min.css (delayed first paint by 387ms)
  • https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.3/css/select2.min.css (delayed first paint by 163ms)
• [ ] Site does not use Githubissues.
• Githubissues is a development platform for aggregating issues.