Bug 14366: Units doesn't get saved usefully for patroncards

Bug 14366: Units doesn't get saved usefully for patroncards

This patch causes the "Units" to be saved and displayed correctly
for the "Edit layout" screen in Patroncards.

_TEST PLAN_

Before applying:
0) Create a new layout
1) Edit the layout, change the units, and click Save
2) Edit the layout again, and notice the units are still "PostScript Points"

Apply the patch:
3) Edit the layout again, change the units, and click Save
4) Edit the layout again, note that the units have changed to your
selection

5) Rejoice

Signed-off-by: Nick Clemens <nick@quecheelibrary.org>
Signed-off-by: Jonathan Druart <jonathan.druart@koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>