Update website to use fetch with keepalive

Companion PR to plausible/analytics#5005

Haven't tested locally!

Author: macobo

_includes/head.html

@@ -18,6 +18,7 @@
     defer
     data-domain="plausible.io"
     src="https://plausible.io/js/script.manual.pageleave.js"
+    data-allow-fetch
   ></script>
   <script>
     window.plausible = window.plausible || function() { (window.plausible.q = window.plausible.q || []).push(arguments) };