ExtensionAuthorsDisplayer should reuse XWiki's standard user picker

The `ExtensionAuthorsDisplayer` contains quite a bit of HTML and inline script to make its user picker work. This was made before the introduction of the velocimacro `#userPicker`. In today's XWiki, this is pretty much just a large amount of tech debt and it should be replaced with the standard picker. The fact that this picker reached this state shows that we can't keep it up to date as things stand.

Before implementation, solving the issue will need a check that there's no missing feature in the userPicker compared to this custom picker. I had a quick look and it didn't look to me like it had anything special implemented but better to be safe than sorry.

As discussed in https://github.com/xwiki/xwiki-platform/pull/5676#event-27188306270

The delete "cross" image in itself is a problem for almost any user: does not use iconTheme, img with an event listener can't be focused by the keyboard, img with an event listener is not conveyed properly by assistive technologies.