ExtensionAuthorsDisplayer should reuse XWiki's standard user picker

I don't think this was a real use case, but the new one still allows showing non-existant users added programmatically (i.e. during import), which is a use case (when some of the listed authors don't have a user profile on the wiki, or it cannot be found at least).

So I guess we need a different jira issue so that we add an optional config to our user picker to allow entering non existing users, right?