Bug 30028: Fix patron message delete confirmation to be translatable

Bug 30028: Fix patron message delete confirmation to be translatable

Make the message deletion confirmation popup translatable. Test plan
uses fr-CA translations.

Test plan:
1) Switch language to french (fr-CA)
If you do not have the translations, go to
  misc/translator
then do
  ./translate install fr-CA
After it is done, go to administration > global system preference >
l18N/L10N then select Français (fr-CA) under language preference.
Refresh then you should be able to switch languages.
2) Add a message to a patron account from your own branch (or make sure AllowAllMessageDeletion is on)
3) Click "Delete" next to the message
--> a confirmation message appears "Are you sure you want to delete this message? This cannot be undone." with options "Cancel" and "OK".
4) Apply the patch
5) Write a translation in misc/translator/po/fr-CA-messages-js.po :
msgid "Are you sure you want to delete this message? This cannot be undone."
msgstr "Êtes-vous sûr de vouloir supprimer ce message? Cette opération est irréversible."
6) Refresh the translations by going to misc/translator and execute ./translate install fr-CA
7) Refresh and click "Delete" again
--> now the confirmation message is "Êtes-vous sûr de vouloir supprimer ce message? Cette opération est irréversible." with options "Cancel" and "OK"

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>