Only ship locale definitions that the user can select in the Welcome Screen

Originally created by @intrigeri on #17139 (Redmine)

While reviewing #17132 (closed), it occurred to me that with #16095 (closed), we’ve restricted the user’s choice wrt. which locale they can login with. But we still install locales-all. Chances are that this includes a large amount of data that cannot be used anymore, so presumably we should:

• either stop installing locales-all and instead use the infrastructure provided by the locales package to generate locale definitions for the locales we make available
• or keep installing locales-all, but delete — via a chroot hook — the locale definitions that cannot be used

But this will increase complexity of our build system, so it would be nice to start with the cheapest possible PoC that can tell us how much this change would shrink our ISO/USB images by. And then we’ll know if it’s worth it.

Related issues

• Related to #17132 (closed)
• Related to #16095 (closed)