Bug 7596 - System prefs editor JS contains untranslatable English string

Bug 7596 - System prefs editor JS contains untranslatable English string

There is one stray instance of an untranslatable string in
preferences.js. This patch replaces it with a variable which is now
defined in the preferences template.

To test, open the system preferences editor and change any setting. You
should see the usual "Saved preference <preference name>" message.

To test that error strings are now translatable, run "perl translate
update <lang>" from misc/translator, then check if the affected strings shows
up in the po file.

Signed-off-by: Liz Rea <liz@catalyst.net.nz>
Tested both, all ok.

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>