Bug 9530: Making changes to C4::Letters

Bug 9530: Making changes to C4::Letters

So notices using it (circulation, holds etc) will now use the new behaviour

To test:

1/ Edit the new systempreferences (ReplytoDefault and ReturnpathDefault)
2/ Optionally edit the branch the mail will be sent from, adding email addresses
3/ Test some mails, a circulation alert, an acquisitions claim, or a newly created borrower alert
4/ Check that the mails have the correct From, Replyto and ReturnPath set
   The rules are
   If the values are set in the branch use that, else use the syspref

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>