[Weblate] get Celery working

Originally created by @hefee on #17347 (Redmine)

/usr/local/weblate$ python3 manage.py check --deploy
SystemCheckError: System check identified some issues:

CRITICALS:
?: (weblate.E020) The Celery is not configured to store results, CELERY_RESULT_BACKEND is probably not set.
        HINT: https://docs.weblate.org/en/weblate-3.5.1/admin/install.html#celery

Weblate itself tells us that something is not correct with Celery ;D

At least we don’t seen anything committed the last 24h, where stuff was modified! That’s why we expect that celery is not working.
But also the knowledge how to get statistics or information out of celery is missing, to check if the others tasks are done by celery.

It seems that we need to configure a CELERY_RESULT_BACKEND - Celery upstream suggests to use a proper broker

At least for me I don’t know what a broker is or why we need a result_backend. So either we find someone who can give us this information or we need to dig into that ourselves.

Related issues

• Related to #17370 (closed)
• Related to #17340 (closed)