Replace celery with crontab

[MagiX13]

Should fix #74

Reduces memory footprint and CPU idle load by removing celery and redis in favor of crontabs as well as moving from the django runserver to gunicorn instead

[l4rm4nd]

Will have to test the cron stuff.

Must say that I have very bad experience using crontab/cron for Django running in a Docker container. This is the whole reason why django-celery-beat is used (best practice + django admin ui configuration possible).

One can also find many issues regarding cron+docker. I just want to ensure a 100% reliable cron for expiry checks.

Thanks for the PR!

[MagiX13]

I read a bit about the docker crontab issues. As this is running crontab as a service in the background, I wouldn't expect much of an issue here to be honest, but then I haven't found the time to test all eventualities here

[MagiX13]

turns out having the correct python path helps running the tasks :)

[l4rm4nd]

Will keep this PR in the backlog.

At the moment, no requirement to switch from celery-beat to plain crontab.

Many thanks though for the PR and the code adjustments!