Revert "set celery default task aware of app context; periodic_task decorator should be used instead of celery's one."
Revert "set celery default task aware of app context; periodic_task decorator should be used instead of celery's one."

This reverts commit bba7e35b33f4318452f0eeda38e97716e9b2f7a5.
but maintains 'periodic_task' in abilian.extensions API

abilian/abilian-core,abilian/abilian-core,abilian/abilian-core,abilian/abilian-core,abilian/abilian-core