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* On the ``SystemWideEnterpriseUserRoleAssignment`` model, adds an ``enterprise_customer`` FK (nullable)
and an ``applies_to_all`` boolean field (defaults to False) that indicates if the user has wildcard permissions.
* Updates the admin to show the "effective" customer in the detail view, and the explicit value in the list view.
The effective value is the deprecated way we currently determine role assignment -
by implicitly assigning the role on every customer to which the user is linked.
* In the detail view/form, the "Enterprise customer" dropdown contains only customers
to which the user is currently linked.