Add a simple method to refresh content when launching or switching tabs

Prevents stale content on load as reported in:
https://fizzy.37signals.com/5986089/collections/2/cards/1449

app/views/pwa/service_worker.js

@@ -1,3 +1,14 @@
+self.addEventListener('fetch', (event) => {
+  if (event.request.method !== 'GET') return
+
+  if (event.request.destination === 'document') {
+    event.respondWith(
+      fetch(event.request, { cache: 'no-cache' })
+        .catch(() => caches.match(event.request))
+    )
+  }
+})
+
 self.addEventListener("push", async (event) => {
   const data = await event.data.json()
   event.waitUntil(Promise.all([ showNotification(data), updateBadgeCount(data.options) ]))