Draft: Implement secure lock screen

When the user is secure locked, all of the user's sessions are frozen and user data is encrypted. Thus, re-authentication must be performed from outside of the session, specifically in GDM.

This MR implements the lock screen that will be presented to the user when their session is secure locked.

Depends on gnome-shell!3258 (which implements the UI for this MR)

Note that with this MR alone nothing will actually happen from the user's perspective. This MR introduces no integration with systemd and homed. Instead, I've split that out into !252 so that this MR can land early