Add PWA support (I think)

static/sw.js

@@ -0,0 +1,13 @@
+self.addEventListener('install', e => {
+    e.waitUntil(
+        caches.open('resendit-v1').then(cache => {
+            return cache.addAll(['/', '/index.html', '/login.html', '/logo.png']);
+        })
+    );
+});
+
+self.addEventListener('fetch', e => {
+    e.respondWith(
+        caches.match(e.request).then(response => response || fetch(e.request))
+    );
+});