For consistency with how normal OOjs UI widgets behave.
Only one usage outside of HTMLForm needed updating. There
are no usages in extensions in Gerrit.
Change-Id: I9b5de86307a13ca7355b7541e8683e4d6a10431f
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
