Put the assets subdomain behind Varnish

- Create a new vhost, 'assets.frontend' (this is what Varnish will connect to)
- Create a new proxy vhost for assets.env.service.gov.uk and make it upstream to Varnish
- Change the assets vhost to listen on the internal hostname rather than the long one
- Add some VCL to tie it all together

I haven't added a healthcheck probe to Varnish for the frontend app servers because Varnish runs on the frontend app servers, and if we can't serve static content then something else is probably a bit wrong.