add constants - ref: https://github.com/learningequality/kolibri/pull/5616#discussion_r290922679
add constants - ref: https://github.com/learningequality/kolibri/pull/5616#discussion_r290922679

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