Add hostname as template param

Steps to reproduce

1. Go to url tab of new external site & add http://{url}

Expected behaviour

Expect to see http://localhost when clicked

Actual behaviour

I'm not allowed to save it with that.

My reasoning is that I'm on a shitty At&t router & the damn thing doesn't resolve my local server at my hostname from my internal network. Which means that when I'm at home the hostname would be 192.168.1.* but when I'm out it's my domain. I'm okay with this behavior, but when I wanna make my own embedded apps for my local server I can't because it won't resolve when I'm outside the network. I would like it so that this would grab the hostname from the request info & embed it that way.

Start in the URL tab flow described in the issue, tracing where external-site URLs are validated and saved and where request information is available. Confirm how template parameters are expanded; done means a URL containing {hostname} can be saved and resolves to the request's hostname when the external site is opened.