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