celery: stop using deprecated celery.decorators We don't use the old magic keyword arguments and can just use celery.task instead - see http://docs.celeryproject.org/en/latest/history/changelog-2.2.html#version-2-2-0

.. _changelog:

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Changelog
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Kallithea project doesn't keep its changelog here.  We refer you to our `Mercurial logs`__.


.. __: https://kallithea-scm.org/repos/kallithea/changelog