PERF: Add more constraint on the Cache Storage usage

Only restricting cache per age wasn't enough for instances with lots of
multimedia usage and high number of posts.

MaxEntries is also more effective on cleanup, and purgeOnQuotaError
advertise that Discourse cache can be purged if necessary.

https://developers.google.com/web/tools/workbox/guides/storage-quota

app/assets/javascripts/service-worker.js.erb

@@ -17,6 +17,8 @@ workbox.routing.registerRoute(
     plugins: [
       new workbox.expiration.Plugin({
         maxAgeSeconds: 7* 24 * 60 * 60, // 7 days
+        maxEntries: 500,
+        purgeOnQuotaError: true, // safe to automatically delete if exceeding the available storage
       }),
     ],
   })