Add ability to route relative to the current URL / browser location API

[daniel-simpson]

Is your feature request related to a problem? Please describe.
I serve homer dashboard on my home server. I usually open it by IP address (e.g. 192.168.1.119), but if I'm outside my network I use tailscale and it's resolution to grab my dashboard (e.g. http://nuc/).

I host multiple other services on this machine on different ports, and would like a way to have homer resolve links relative to my current URL (either http://192.168.1.119 or http://nuc).

Describe the solution you'd like
A concise template for generic services that allows some or all of the url property to be parameterised, e.g.:

- name: "Relative URL example"
  icon: fa-explosion
  subtitle: A link hosted on the same server as Homer, but with more than one possible hostname
  url: "//${hostname}:8080

Describe alternatives you've considered
PR incoming, happy to make adjustments or discuss further.

Additional context
N/A