Bring Users UI module in line with Items UI module

Because John C did the scaffolding for the Items UI module some time after that for Users, it's more advanced in several respects, notably in the capabilities of the MultiColumnList component. Before I can start factoring common code out of these two UI modules (STRIPES-172), I will need to get Users up to speed, using the same facilities as Items. That will also have concrete benefits, such as highlighting in the results list the item that has been selected for the main panel.