add dovecot cleanup timer to purge mdbox files

Merged Frederik Schwan requested to merge freswa/infrastructure:add-dovecot-cleanup-timer into master

While mbox and maildir files get deleted by dovecot right away, that's not the case for mdbox files. Since they contain multiple mails at once in a proprietary format rewriting is expensive. That's why this step is done in a separate step outside the dovecot process.

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