Background processing with Celery

[luytena]

Certain tasks in caluma can currently lead to timeouts when performed with large amounts of data. One example is the recalculation of calculated answers when modifying the calc expression of a calculated question:

caluma/caluma/caluma_form/signals.py

Lines 104 to 114 in 4ea87d0

	@receiver(post_save, sender=models.Question)
	@disable_raw
	@filter_events(lambda instance: instance.type == models.Question.TYPE_CALCULATED_FLOAT)
	def update_calc_from_question(sender, instance, created, update_fields, **kwargs):
	# TODO optimize: only update answers if calc_expression is updated
	# needs to happen during save() or __init__()
	for document in models.Document.objects.filter(form__questions=instance):
	update_or_create_calc_answer(instance, document)

Tasks such as this (there might be other examples as well) would benefit from moving them into a background task using Celery.

[open-dynaMIX]

Related to #2339.