Convert data cleanup command to database setting

[dennissiemensma]

Originally reported by: Dennis Siemensma (Bitbucket: dennissiemensma, GitHub: dennissiemensma)

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The management command for cleaning up data should be configurable as a setting. We should also look for an app to schedule tasks, as we also need to export data periodically for external API's.

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• Bitbucket: https://bitbucket.org/dennissiemensma/dsmr-reader/issue/13

[dennissiemensma]

Original comment by Jeroen Peters (Bitbucket: jeroenpeters, GitHub: jeroenpeters):

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In the description, you state We should also look for an app to schedule tasks, there isn't a ticket/issue for this, am I right?
It might be a nice 'But first' for #13 and #10

[dennissiemensma]

Original comment by Dennis Siemensma (Bitbucket: dennissiemensma, GitHub: dennissiemensma):

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Yeah agreed. This was before integrating django-solo + signals, so it's kinda outdated but still required for the 1.0 release (which should contain enough tools). Low prio though.

[dennissiemensma]

Issue is old and was migrated from bitbucket. Created a fresh one here: #72