Add ability to configure how to display a user name

Right now, the default is hardcoded in XWiki#getUserName() and is "firstName lastName". However, this is not good for some languages where last name should be first.

Thus, the idea is to make this configurable in the Admin UI, by proposing the ability to use a velocity script to control the display logic. Note that getUserName() already supports executing a velocity script but it's not used ATM.

See:

• 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/13068/2