Deprecate old client API with warnings.
Deprecate old client API with warnings.

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