bug 2848 [1/2]: Send notifications to patrons when hold is received

bug 2848 [1/2]: Send notifications to patrons when hold is received

This adds notification code to Reserves.pm to send notifications through email
or SMS to patrons when their hold is filled. It does not send the letter
directly, but instead uses EnqueueLetter. Also, it relies on
EnhancedPatronMessaging being on and the hold notification being turned on for
the specific patron through the 'Messaging' tab.

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>