Bug 11742: Change return type for GetLetters

Bug 11742: Change return type for GetLetters

The GetLetters subroutine should return an arrayref with different
letters for a module.

Test plan:
0/ Delete your notices with module=claimacquisition, claimissues,
serial
1/ Go on the late orders page (acqui/lateorders.pl) and verify you
cannot choose a notice for claiming
2/ Create a notice with module=claimacquisition
3/ Go on the late orders page (acqui/lateorders.pl) and verify you
can choose the notice for claiming
4/ Go on the Claim serials page (serials/claims.pl) and repeat the same
thing with the a "claimissues" notice
5/ Create a new subscription (serials/subscription-add.pl) and verify
you cannot choose a notification for patrons.
6/ Create a notice with module "serial" and verify you can.

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Passes all tests and QA script. Additional tests done:

- copy notice ODUE, on saving you are now prompted to choose
  a new CODE for the notice
- edit new notice, try to set code back to ODUE. You are
  prompted that the code is already in use.

This will prevent people from accidentally overwriting a letter
with the same letter code.