Add factory reset

This should call schnapps rollback factory and reboot.

The idea is that right now when user wants to do factory reset it needs to go to router directly. With this it would be possible to do the same from Foris.

Note that schnapps has to be available and root has to be BTRFS (should we extend schnapps to check that current root is managed by schnapps?). It has to be signaled somehow to frontend that factory reset is available because of that.

Supporting it on Turris 1.x without BTRFS is not required.