merge web app manifests (#708)

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

index.html

@@ -15,7 +15,7 @@
   </link>
   <link href="public/favicon/favicon-16x16.png" rel="icon" sizes="16x16" type="image/png">
   </link>
-  <link href="public/favicon/site.webmanifest" rel="manifest">
+  <link href="public/manifest.webmanifest" rel="manifest">
   </link>
 
   <link color="#5bbad5" href="favicon/safari-pinned-tab.svg" rel="mask-icon">