Update CHANGELOG for 0.12.6
Update CHANGELOG for 0.12.6

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