fix: cache font file on-demand

src/service-worker.js

@@ -38,6 +38,7 @@ const assets = __assets__
   .filter(filename => !filename.endsWith('.webapp')) // KaiOS manifest
   .filter(filename => !/emoji-.*?\.json$/.test(filename)) // useless to cache; it already goes in IndexedDB
   .filter(filename => !/screenshot-.*?\.png/.test(filename)) // only used during PWA installation, don't bother caching
+  .filter(filename => !/TwemojiCountryFlags\.woff2/.test(filename)) // cache on-demand
 
 // `shell` is an array of all the files generated by webpack
 // also contains '/index.html' for some reason