The RelatedUserWidget now supports selecting either a single value

(new behaviour) or multiple values (old behaviour & the default) through

a new parameter. When in single-valued mode, the widget will be rendered

without the list of item selections and will use selectize.js's renderd

<select> as the only selected item. When in multi-valued mode, the

widget will be rendered as before.

Used this with an upcoming patch for the OAuth form.
Tested the user selector on the repository admin form and ensured

  that multi-valued selections still work.