Right now here are 2 web app manifests:
- public/favicon/site.webmanifest
- public/manifest.json
But only the 1st one is referred in the html
This commit
- merged them into public/site.webmanifest (according to https://developer.mozilla.org/en-US/docs/Web/Progressive_web_apps/Add_to_home_screen#manifest, it should be in the root, whereas the spec suggests the `.webmanifest` extension
- complete the required fields
It now passes the Lighthouse PWA manifest test in Chrome
