Bug 11742: A letter code should be unique.

Bug 11742: A letter code should be unique.

This patch is a dirty way to fix a design issue on notices.
Currently the code assumes that a letter code is unique. Which is wrong,
the primary key is module, code, branchcode.

Maybe we should add a primary key (id) for the letter table in order to
pass the id to the template and correctly manage the letter code
duplication.

Test plan:
Try to duplicate a letter code using edit, add and copy actions.
If you manage to do it, please describe how you did.

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>