types.string is deprecated because it quietly concatenates strings

Issue description

types.str should be used instead of types.string

What happened:

building Nix...
building the system configuration...
trace: warning: types.string is deprecated because it quietly concatenates strings
updating GRUB 2 menu...

Technical details

 - system: `"x86_64-linux"`
 - host os: `Linux 5.2.9, NixOS, 19.09pre189585.1412af4b2cf (Loris)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.2.2`
 - channels(root): `"nixos-20.03pre193701.262b328b0ba"`
 - channels(josephtheengineer): `""`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

SNM Version

(master)[https://gitlab.com/simple-nixos-mailserver/nixos-mailserver/-/archive/master/nixos-mailserver-master.tar.gz] commit ab33e87c

Relevant part of the config to reproduce:

        mailserver = {
                enable = true;
                fqdn = "mail.theengineer.life";
                domains = [ "theengineer.life" "josephtheengineer.ddns.net" "qgt.ddns.net" ];

                # A list of all login accounts. To create the password hashes, use
                # mkpasswd -m sha-512 "password"
                loginAccounts = {....};

                # Extra virtual aliases. These are email addresses that are forwarded
                extraVirtualAliases = {....};

                # Use Let's Encrypt certs. Note that this needs to set up a stripped down 
                # nginx and opens port 80.
                certificateScheme = 3;

                # Enable IMAP and POP3
                enableImap = true;
                enablePop3 = true;
                enableImapSsl = true;
                enablePop3Ssl = true;

                # Enable the ManageSieve protocol
                enableManageSieve = true;

                # Scan inbound emails for viruses (NOTE: This requires at least 1GB RAM)
                virusScanning = true;
        };