Add E-Mail notification for due date

[rabesocke]

Is your feature request related to a problem? Please describe. We have several people in our club who rarely use Nextcloud - unfortunately. Most of the time we have to remind them of their cards because of an overdue due date.

Describe the solution you'd like There should be a notification and a E-Mail to the assigned users with a list of cards where the due date is near or overdue.

Describe alternatives you've considered E-Mail notifications via the calendar is also possible (#15), but a direct E-Mail notification from Deck would be better in order to avoid messing up the calendars.

Additional context #566 and #414

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