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

For the second part:

Even better allow to control the format of the displayed user information, for examples :

By default : $firstName $lastName
$lastName $firstName
$firstName $lastName ($userName)
...

See XWIKI-22277

We'll need to decide if we want to keep the qualifier idea that was previously implemented or remove it (we can probably still do it as the code is stil unstable as it was introduced in 15.5, but we don't have much time). I also believe that using a configurable display (using some cached velocity script) is the most flexible way.