Optimize nxu:userFullName function

Currently nxu:userFullName function does the login/display name resolution using directly UserManager.

the problem

Unfortunately, calling UserManager.getPrincipal(username) can be too costly :

• In case of LDAP groups + Slow LDAP fetching the references will be slow (and not cachable)
• Computed Groups can take time

There is a small cache directly inside the JSF Context but it is on a per user session basis.

Solutions

In parallele of the patch implementation that is here for backport reasons, we may provide 2 news API in UserManager :

• getUserFullName();
• getGroupLabel();

This way we can hide the caching/invalidation logic directly inside the UserManager and avoid doing complex code inside the JSF functions !