mailman3-web needs Q_CLUSTER retry setting

Installation of mailman3-web (0+20200530-2.1) on mantic produces this warning:

    Creating config file /etc/mailman3/mailman-web.py with new version
    /usr/lib/python3/dist-packages/django_q/conf.py:139: UserWarning: Retry and timeout are misconfigured. Set retry larger than timeout,
            failure to do so will cause the tasks to be retriggered before completion.
            See https://django-q.readthedocs.io/en/latest/configure.html#retry for details.

This refers to the 'retry' and 'timeout' fields of the Q_CLUSTER dictionary configured in /usr/share/mailman3-web/settings.py.

That file sets 'timeout' to 300, but has no setting for retry. The documentation linked above states that the built-in default is 60 seconds.

Though Django reports this as a warning, clearly this configuration will cause bugs such as tasks being executed multiple times (up to 5 retries could be started while previous tasks may still finish).

The fix is to add a 'retry' setting to the settings file. Assuming that the 300s timeout was deliberately chosen upstream, then setting the retry interval to 360 would be sensible (= retry one minute after timeout). A patch is attached.