Bug 36915: Send email notification when a booking is cancelled

Bug 36915: Send email notification when a booking is cancelled

When a booking is cancelled, patron received an email based on specific
letter.

Test plan:

1) Create a letter with code "BOOKING_CANCELLATION" on "Notices and
   slips" tool.
2) Add booking or go on item already booked in advance.
3) Cancel it
4) Verify in message_queue table directly or go on patron page and click
   on "Notices" tab section.

Sponsored by: Association de Gestion des Œuvres Sociales d'Inria (AGOS)
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>