add optional slot for checkbox
add optional slot for checkbox

indirectlylit/kolibri,learningequality/kolibri,mrpau/kolibri,learningequality/kolibri,mrpau/kolibri,mrpau/kolibri,learningequality/kolibri,learningequality/kolibri,mrpau/kolibri,indirectlylit/kolibri,indirectlylit/kolibri,indirectlylit/kolibri