See https://forum.xwiki.org/t/new-basic-user-api/6329 and https://design.xwiki.org/xwiki/bin/view/Proposal/UserAPI
The goal is to add a basic API so that we can get started.
Rely on the new XWiki User API
Superadmin user should see hidden documents by default
Provide an API to check if a reference is an existing user
Introduce new URL type for users
Add ability to define a custom URL for a user profile
Introduce and send UserCreating/UserCreated/UserUpdating/UserUpdated/UserDeleting/UserDeleted events when a user is created/modified/deleted