config(nixos): Add Mullvad VPN to the system-wide configuration

I am already paying for it to use it on my phone, why not use extra slot for
the laptop? Also install "mullvad-browser", which is presumably optimized
for privacy by somebody who is paid to stay on top of all dirty survelliance
tricks. I don't have energy to do it myself, so let's call it "good enough".

Unfortunately, the way mullvad-vpn service manages the network interfaces
breaks my openvpn connection to $dayjob virtual machines, so I can't use it
during working hours. Alright, that draws a line, which might be a good thing.

I'll fix it eventually.

config/nixos/configuration.nix

@@ -11,6 +11,7 @@
     ./hardware-configuration.nix
     ../../home-manager/nixos
   ];
+  services.mullvad-vpn.enable = true;
 
   home-manager.users.kaction = import ./user-kaction.nix;
   home-manager.useGlobalPkgs = true;
@@ -166,6 +167,8 @@
     lagrange
     alejandra
     (zathura.override {plugins = [zathuraPkgs.zathura_pdf_poppler zathuraPkgs.zathura_djvu];})
+    mullvad-vpn
+    mullvad-browser
   ];
 
   boot.kernel.sysctl."net.ipv4.ip_unprivileged_port_start" = 0;