public void setSelectionChangeConsumer(final Consumer<UserRef> selectionChangeConsumer) {
        this.selectionChangeConsumer = selectionChangeConsumer;
    }
For selecting a username to add a user to a permissions set or selecting a user to add to a group.
For internal IDP this will list users from account and stroom_user combined.
For external IDP this will list users from stroom_user only and users on the IDP but not in
stroom_user will have to be manually added to the list.
stroom-core-client/src/main/java/stroom/security/client/presenter/UserRefPopupPresenter.java