feat(mail): Import email configuration into Nix

This moves my email configuration into NixOS, including the following
features:

1. Replaced the NixOS-builtin offlineimap user service with a custom
   one that runs notmuch-indexing as part of the systemd unit instead
   of a postsynchook, which is significantly more reliable.
2. Adds configuration for notmuch and its tagging.
3. Adds configuration for OfflineIMAP & MSMTP.

Relevant emacs configuration has also been added to my emacs.d
repository.

configuration.nix

@@ -7,11 +7,12 @@
 {
   imports =
     [
-    ./hardware-configuration.nix
-    ./local-configuration.nix
-    ./packages.nix
     ./desktop.nix
     ./dotfiles.nix
+    ./hardware-configuration.nix
+    ./local-configuration.nix
+    ./mail.nix
+    ./packages.nix
     ];
 
   # Use the systemd-boot EFI boot loader.
@@ -61,12 +62,6 @@
   # Enable GNOME keyring (required for Evolution)
   services.gnome3.gnome-keyring.enable = true;
 
-  # Enable OfflineIMAP (configuration itself is user-data)
-  services.offlineimap = {
-    install = true;
-    path    = with pkgs; [ pass notmuch ];
-  };
-
   virtualisation = {
     # Configure Docker (with socket activation):
     # Side note: ... why is this in virtualisation? ...