To use OOUI for forms with user name autocomplete, the new widget
UserInputWidget interacts like the jQuery pendant (working with css
class "mw-autocomplete-user").
It is also available in HTMLForm as "user".
Example usage: Iaeff912e6437d6ebef0d5b1919ce8cf53a7fd5f1
Change-Id: I9501c85f4288c255bbe3a5284e99b57b6169916f
