Add custom URL prefix for HTTP URLs

[rechner]

Hello 👋

A small tweak to the way that URLs for album art is generated for the HTTP RPC server. I utilize the Snapweb interface through a reverse proxy with SSL termination, which unfortunately breaks album art images in snapweb due to port mismatch and SSL mixed content policies (e.g.: my snapserver hosted at snapserver.lan on the default port, is available over the WAN via a reverse proxy at https://snapcast.example.com, but setting http.host = snapcast.example.com results in image URLs being http:https://snapcast.example.com:1780).

I worry this addition might create some confusion in the configuration with the http.host configuration, but allowing a totally custom URL prefix like this provides better flexibility of schema, hostname, and port instead of using a piecemeal-constructed string, does it make sense to keep both around?

Tested on Debian 11.

Thanks,

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[badaix]

Thanks!
Merged it into the tls branch, because this must be touched in the TLS branch anyways.