Invert logic of reminder_days appconfig to be relative to the actual expiration duration

[nickvergessen]

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Changing the expiration can happen (e.g. from 9 to 6 months), but reminders being send 4 and 2 weeks before would be "targeted" 8 and 8.5 months as they are not adjusted. So having them set to 14 and 28 days instead and that being automatic relative to the 9 or 6 months is much better handle-able