fix: clear a stale organisation_role when a user's organisation changes in the admin

The members relation manager and the organisation field of the Users form
declare cabana ClearOnChange on organisation_role: adding or removing a
member, and a form save that moves a user to another organisation or to
none, clear the role unless the same save writes it. Needs a SummerCMS
release with ClearOnChange.

controllers/users_admin_controller.go

@@ -364,7 +364,9 @@ const groupsField = "groups"
 // AdminFieldRelations binds the organisation picker and the groups field.
 // organisation_id is a protected column, so the contract opts in to writing
 // it through this one field; the submitted id is still checked against the
-// options. groups is the users_groups pivot: a save that carries the field
+// options. A save that changes the organisation (to another one or to none)
+// clears organisation_role (ClearOnChange) unless the same save writes it.
+// groups is the users_groups pivot: a save that carries the field
 // replaces the user's memberships, and AdminRelationLocks decides which of
 // them the administrator may change.
 func (usersAdminController) AdminFieldRelations() []cabana.FieldRelationContract {
@@ -376,6 +378,7 @@ func (usersAdminController) AdminFieldRelations() []cabana.FieldRelationContract
 			ForeignKey:         "organisation_id",
 			LabelColumn:        "name",
 			WritableForeignKey: true,
+			ClearOnChange:      []string{"organisation_role"},
 		},
 		{
 			Field:             groupsField,