3.17.14

• On the SystemWideEnterpriseUserRoleAssignment model, adds an enterprise_customer FK (nullable).
• 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.
• If adding a new assignment via the admin form, the customer dropdown will contain no customers to choose from at all - the administrator must first provide a user and role, click "Save and continue editing", and then a list of linked customers will be present. The help text of the field mentions this. Rationale: we don't want to provide a drop-down list of every customer that exists in production when creating a new assignment. The alternative would be to make this form more "dynamic", which would require more development work than provided value.

#1117