Adds also periodic tasks with django-celery-beat.

Note that as of now, it requires 3 services (to do with Celery) to run:
celery -A SciPost_v1 worker --loglevel=info -E
celery -A SciPost_v1 beat --loglevel=info --scheduler django_celery_beat.schedulers:DatabaseScheduler
And optionally (for using the 'flower' console for monitoring tasks -
might not use this after all):
celery -A SciPost_v1 flower

Additionally, the 'rabbitMQ' broker needs to be installed - working on
it on the staging at the moment.

I will add this in some proper config/readme later.