Add ability to define a custom URL for a user profile

Right now, users are stored in wiki pages inside of XWiki. However, we could imagine users defined externally and having profile pages defined externally. In this case clicking on their names inside of XWiki should point to these external URLs. Thus we need a way to be able to define an external URL for a user. When not specified, it'll default to a profile URL inside of XWiki.

See https://forum.xwiki.org/t/user-api-and-urls/6467

I've proposed to define a "profileURL" user property for that.