Add Celery registration module about foca

[uniqueg]

For this, I think we can really only migrate the register_celery.py module. The background tasks are intrinsically service-dependent.

[kushagra189]

I was going through the utils part and I thing the state variable will be a hinderence. So for now I'll stick to the addition of register_celery part.

[uniqueg]

I think I will take care of this together with #31, based on PR #39. It's getting really tricky here

[uniqueg]

Actually, we can't use the TaskMonitor either, so what's left are just these calls:

celery_app = create_celery_app(app)
logger.info('Celery task monitor registered.')

They just fit very well into the main foca(), so won't fix.