Remove dependecies from celery_once customizations

Situation

Alliance Auth is using the third party library celery-once, which allows to prevent multiple execution and queuing of tasks. As part of the configuration it has defined it's own QueueOnce class with custom defaults, and it's own backend for locking. QueueOnce is a popular feature in Alliance Auth and community apps.

Problem

The customization implementations are currently located in allianceauth.services.tasks, where they are mixed up with other task related components. Those other components require Django to be fully up and running to function (e.g. the User model needs to be available). This creates an unnecessary dependency and e.g. prevents celery-once from being used during Django startup.

While removing unnecessary dependencies always is a good thing, this dependency specifically prevents me from using celery once in one of my apps.

Solution

This change removes that dependency by putting all celery once related components into their own package (allianceauth.services.celery_once). The external interface - i.e. the QueueOnce class - remains available from allianceauth.services.tasks to ensure backwards compatibility. Note that admins will need to adjust their local celery configuration (myauth/celery.py) for this change as follows:

...
app.conf.ONCE = {
    'backend': 'allianceauth.services.celery_once.backends.DjangoBackend',
    'settings': {}
}
...