* Add a failing test
This is failing because sponsorships are not removed by Users::Delete,
and raises a foreign key constraint error because user.destroy tries
to remove a referenced user.
* When destroying a user, destroy associated sponsorships
There is a `belongs_to` relationship in sponsorships, and a has_many
relationship in Organization, this adds a mirror relation on User with
the equivalent dependent destroy rules.
If an organization is removed, all sponsorships belonging to that
organization are removed too, and now, if a user is removed, all
sponsorships created by that user are removed.
Co-authored-by: djuber <djuber@forem.beta-reduction.com>
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