nixos/bookstack: Make the hostname configurable...
nixos/bookstack: Make the hostname configurable...

...and set a reasonable default `appURL` based on it.

This is pretty much required when configuring ACME, and useful in
general.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs