Add the possibility to distinguish different accounts of the same user on the UI

Note that my suggestions that it should be possible to, for example, reverse first and last name or display a different attribute are actual user demands:

• https://forum.xwiki.org/t/how-to-customize-user-name-display/3492
• https://forum.xwiki.org/t/any-way-to-change-xwiki-username-display-format-to-use-the-fullname-attribute-instead-of-first-name-last-name/13068

We could of course see this as a separate demand, I'm just saying that a more general "format string" could fulfill these real user demands, too, without increasing complexity.