Introduce new URL type for users

Continuing with our work to make users independent of the storage implementation in wiki pages, we also need a new URL type for accessing user profiles so that we don't use the "bin" or "wiki" URL type which is reserved for wiki pages.

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

The current proposal is:

<URL domain part>/xwiki/user/[wiki/]<username>

Examples:

https://localhost:8080/xwiki/user/Admin
https://localhost:8080/xwiki/user/subwiki/Admin