filter out non-available exercises in has_exercise filter
filter out non-available exercises in has_exercise filter

mrpau/kolibri,DXCanas/kolibri,lyw07/kolibri,mrpau/kolibri,mrpau/kolibri,indirectlylit/kolibri,learningequality/kolibri,lyw07/kolibri,DXCanas/kolibri,learningequality/kolibri,DXCanas/kolibri,lyw07/kolibri,indirectlylit/kolibri,indirectlylit/kolibri,learningequality/kolibri,indirectlylit/kolibri,mrpau/kolibri,DXCanas/kolibri,learningequality/kolibri,lyw07/kolibri