Set worker log retention to 30 days

[eloquence]

Description

The logs in /var/log/securedrop_worker are currently set to rotate at a size of 50 MB. While these logs only include functions and filenames, to minimize accumulation of metadata (and to future-proof logging behavior), we should enforce a retention period, at most 30 days, consistent with the discussion in #304 and #3714. This will require adding a logrotate configuration and disabling the more limited rotation provided by supervisord.

Weekly rotation may be appropriate given the typically low event volume.