nixos/offlineimap: Move to services/mail

The services/networking directory is already quite polluted and the
first point where I was looking for the offlineimap module was in
services/mail and didn't find it there.

Offlineimap already has IMAP in its name and clearly belongs to the
"mail" category so let's move it there.

Tested by evaluating a configuration with services.offlineimap enabled.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @DamienCassou

nixos/modules/module-list.nix

@@ -210,6 +210,7 @@
   ./services/mail/freepops.nix
   ./services/mail/mail.nix
   ./services/mail/mlmmj.nix
+  ./services/mail/offlineimap.nix
   ./services/mail/opendkim.nix
   ./services/mail/opensmtpd.nix
   ./services/mail/postfix.nix
@@ -382,7 +383,6 @@
   ./services/networking/ntopng.nix
   ./services/networking/ntpd.nix
   ./services/networking/nylon.nix
-  ./services/networking/offlineimap.nix
   ./services/networking/oidentd.nix
   ./services/networking/openfire.nix
   ./services/networking/openntpd.nix