GDM: Change setting based on selected user

This is something I've been thinking about more and more as I continue working on homed. I'm envisioning this would be a great UX:

1. You select a user in GDM
2. GDM changes around its language, light/dark mode setting, 24 hour time settings, etc all using the data stored in the user record. Maybe even monitor settings 👀? See #33
3. The user-specific page has all the various user settings pre-applied to it and thus fits in with the user.

As for the main GDM page, it either has some system defaults (using the system locale, etc). Or it uses settings from the last-logged-in user?