Include djangoapps.heartbeat by default

If this isn't installed, the sample_task provided by
openedx.core.djangoapps.heartbeat isn't imported, and thus are never
registered with the celery workers.  This sample_task is used for the
extended heartbeat api call. Since the celery heartbeat check is turned
on by default, we should also make sure this djangoapp and thus
sample_task is registered by default. Otherwise the extended heartbeat
check fails with the default configuration.