celery: upgrade to Celery 4

Celery 3 doesn't support Python 3.7 or later. This upgrade is thus essential
for full Python 3 support. But note that
https://docs.celeryproject.org/en/4.4.0/faq.html#does-celery-support-windows
says "No".

The names of config settings changed in Celery 3 to 4, as described on
https://docs.celeryproject.org/en/3.0/whatsnew-4.0.html#lowercase-setting-names .

Celery 4 config settings can now be specified in Kallithea .ini files by prefixing
with `celery.` - for example as `celery.broker_url`.

Remain backwards compatible for the usual settings, and map old names to the
new names.