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

Hey all,

this issue was highlighted as part of a [forum discussion|https://forum.xwiki.org/t/how-to-hide-personal-information-in-xwiki-with-public-visibility/16575/5]. I'd like to add my 2 cents here:

The full name (given and family name) is considered personal data under GDPR which is applicable to all European countries. Imagine a wiki instance which is making use of centralized user management (doesn't matter if it's LDAP, OIDC, SAML, or whatever) and therefore gets the user information from the identity provider. On top of that, the wiki has public pages which are not limited to internal people but visible to unauthenticated users.

The full name of users show up on various different places. In their profile, in the page header (last modified by...), in the page history, and probably quite a few other places that I'm missing currently. At a minimum I'd opt for a configuration setting that allows to configure how user names are displayed, even better would be a flexible solution which can control the username display behavior based on context (e.g. show the name to another logged in user, but hide the full name from an unauthenticated user).