Refactor Users UI module to use generic MultiColumnList component

In STRIPES-160 (React-component scaffolding for item list), John C. mentions "an updated version of the MultiColumnList component that exposes new props for row and header click events as well as selective column rendering/ordering and formatting of data."

The Users module currently uses its own fork of MultiColumnList, ui-users/lib/MultiColumnList/MultiColumnListUsers.js. I would like to get rid of that and migrate its functionality back up into the master MultiColumnList – hopefully it is now powerful and flexible enough for that to be possible.