Bug 14141: Do not let edit the branch when updating/copying notice

Bug 14141: Do not let edit the branch when updating/copying notice

If a user modify the library when he's updating a notice, the notice
will be duplicated, instead of modified.

To reproduce:
1/ Go on the notice & slips tools (tools/letter.pl)
2/ Create a notice A for library L1
3/ Edit this notice and update the library with L2
4/ On the list view, you will observe that 2 notices A now exist, one
for L1 and one for L2.

The code in tools/letters.pl should be rebuilt completely, it becomes a
house of cards. To prevent this bug, I suggest to not let the user
modify the library value on editing a notice.
It cans be done by copying the notice to another library and remove the
original one.
Counter patch welcomed.

Test plan:
1/ Go on the notice & slips tools (tools/letter.pl).
2/ Create a notice A for library L1.
3/ Edit this notice: you are not able to modify the library anymore.
4/ Copy the notice to library L2. You are not able to modify the library
neither.

Works ok.

Signed-off-by: Amit Gupta <amit.gupta@informaticsglobal.com>
Signed-off-by: Nick Clemens <nick@quecheelibrary.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>