Create an ActivityPub user picker

AP relies on WebFinger or on AP actor URLs to send messages, so we have to use them for interactions. However, once a user interacted with an instance, it is stored in the DB so we could reuse the data we already have to propose a user picker dedicated to ActivityPub.

Some information about pickers on XWiki: https://www.xwiki.org/xwiki/bin/view/Documentation/DevGuide/FrontendResources/AutoSuggestWidget/#HSuggestUsers

A simple way for doing it would be to request all actors matching names in AP storage.