Screen locker does not work in Buster
The screen locker in Buster based Tails versions does not work.
When an administration password is set, neither pressing Super+L or clicking the button in the system menu will lock the screen.
When no administration password is set and pressing Super+L or clicking the screen locking button in the system menu will show the prompt for setting a password for the screen locker. When "Lock Screen" is clicked after a password was specified nothing will happen.
Fix screen locker not working in Buster (refs: #16763)
We used the disable-lock-screen dconf setting in order to hide the
default lock screen button in the system menu. But in Buster, this also
disables locking the screen via the org.gnome.ScreenSaver D-Bus service,
so we can't use this setting anymore.
This commit disables the disable-lock-screen dconf setting and hides the
default lock screen button via out gnome-shell extension instead.