Bug 12532: Send email to guarantee and guarantor

Bug 12532: Send email to guarantee and guarantor

This patch allows guarantors to receive emails sended to their
guarentees. This patch is a rebase of the previous patches.
I took all the content of previous commit and put it in one commit.

TO TEST:

Before applying:
1) Search, or create, a patron with guarantor.
2) For both guarantors and guarantees:
   - Add an email address
   - Update the 'Patron messaging preferences' section so that an email is sent for item checkouts
3) Checkout an item. An email should be sent only to the guarantee.
4) Apply the patch.
5) Run updatedatabase.pl
6) Run prove t/db_dependent/Members.t and prove t/db_dependent/Letters.t
7) Enable 'RedirectGuaranteeEmail'
8) Run misc/cronjobs/process_message_queue.pl
9) Notice that the email should be sended to both the guarantee AND the
guarantor.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>