Configure localed to run locale-gen to generate missing locale

Hi! This change applies an (upstreamed) patch to permit systemd-localed to run locale-gen to generate missing locale when requested to switch the system locale. This makes localectl usable on Debian for changing locale without breaking system localization or even prevent certain applications from running at all after switching to a non-generated locale (one such example is gnome-terminal, which won't run if the system has a default-locale set that isn't generated).

The patches were reviewed upstream and have recently been merged into systemd's master branch for the next release - but we likely do want them in Debian sooner, for the next Debian release. In Purism, we have also been using versions of this patch for a while to make gnome-initial-setup & Co. work, so this change has already received quite a bit of testing in PureOS.