Add PWA support (I think)

templates/index.html

@@ -6,6 +6,12 @@
     <title>Send.it</title>
     <script src="https://cdn.tailwindcss.com"></script>
     <link rel="icon" type="image/x-icon" href="/static/favicon.ico">
+    <link rel="manifest" href="/manifest.json">
+    <script>
+        if ('serviceWorker' in navigator) {
+            navigator.serviceWorker.register('/sw.js');
+        }
+    </script>
     <style>
         * {
             border-radius: 0 !important;