NixOS module configuration improvements

[quentinmit]

This branch has three different configuration improvements. The first is that it provides declarative configuration for system settings, in addition to the config.yaml file. This can be used, for instance, to enable SSO login:

services.inventree = {
  config.social_backends = [
    "allauth.socialaccount.providers.openid_connect"
  ];
  systemSettings.LOGIN_ENABLE_SSO = true;
  systemSettings.LOGIN_ENABLE_SSO_REG = true;
  systemSettings.LOGIN_ENABLE_PWD_FORGOT = false;
};

The second improvement is to restore the recently-deleted serverBind option; it was replaced with bindIp and bindPort settings, but the latter cannot be used to specify a Unix socket such as:

services.inventree = {
  serverBind = "unix:/run/inventree/inventree.sock";
}

Finally, I replaced the siteUrl and allowedHosts settings with config.site_url and config.allowed_hosts respectively. Having the settings as environment variables was causing the default values to override the explicit settings I had put in config; now, they are only set in one place. (I used lib.mkRenamedOptionModule to not break any existing configurations and allow the old option name to continue working.)