Bug 13613: Don't allow digest to be selected without a digest-able transport selected

Bug 13613: Don't allow digest to be selected without a digest-able transport selected

By default, only the email transport is enabled. This gives the
messaging preferences the look of having email and digest and two
options. I.E. to some users it appears that you check email for single
emails, *or* you check digest for a digest email.

To help remove this possible confusion, the digest mode checkbox should
be disabled if no digest-able transports are checked for a given notice.

Test Plan:
1) Apply this patch
2) For both the staff interface and the opac, note that the digest
   checkbox is disabled any time that no digest enabled transport (
   email, sms if enabled ) is checked for the patron editor.

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>