Adds http2 preload headers to be send to the server
In the template, add new prefetches with
<html>
<body>
<img src="{% pushed_static 'my_image.png' preload_as image %}"/>
</body>
</html>
it will render to
<html>
<body>
<img src="my_image.png"/>
</body>
</html>
and add the link headers in the middleware:
Link: "<my_image.png>; as=image; rel=preload"