tty2-session.post: Use systemctl to enable getty@tty2

This always works, whatever is the systemd unit path.
Replace maui-kf5 and maui-plasma5 repositories with maui-frameworks
x86-livecd.yaml: Add maui-installer to the packages

maui-installer contains modules and configuration for Calamares
therefore pulls it as a dependency.

We keep installer-shell temporarily but as soon as next version is
ready we are gonna remove it completely.