extensionSystem: Add kill-switch setting to disable user extensions

When gnome-shell fails to start on login, gnome-session tries to
re-launch it again with all extensions disabled. This is currently
implemented by clearing the list of enabled extensions, which means
the user needs to re-enable their extensions manually again.
To make this process less annoying, add a single 'kill-switch' setting
gnome-session can use without interfering with the user setting.

https://bugzilla.gnome.org/show_bug.cgi?id=778664