Set a different lock screen background by default

We are using the Sunset background that is shipped with
gnome-backgrounds as the default lock screen background.

https://bugzilla.gnome.org/show_bug.cgi?id=706368
[l10n] Updated Italian translation.
Updated Norwegian bokmål translation
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scemas: Add datetime schema

For now, this just contains a 'automatic-timezone' key, which is needed
for the Date & Time panel in gnome-control-center.

https://bugzilla.gnome.org/show_bug.cgi?id=706980
Updated Galician translations
[l10n] Updated Italian translation.
Updated Brazilian Portuguese translation
Updated Polish translation
Updated Lithuanian translation
3.9.91
Updated Spanish translation
Updated Slovenian translation
Updated Slovenian translation
Updated Czech translation
Updated Traditional Chinese translation(Hong Kong and Taiwan)
Updated Serbian translation
Updated Latvian translation
Updated Indonesian translation