Remove boards when a user gets deleted

[juliusknorr]

Once a user is deleted from the Nextcloud instance all his personal boards should be removed. Boards that are shared should ideally be transfered to another user who has managing permissions on the board.

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[arnowelzel]

BTW: This can also lead to the infamous "Call to a member function getUID() on null" errors in the cron log - see nextcloud/server#9953. So I would even see this as a bug and not only an enhancement.

Also see the documentation about event hooks in Nextcloud:
https://docs.nextcloud.com/server/13/developer_manual/app/hooks.html

[juliusknorr]

Fixed with #1976