Move User and Group displays into a macro

Move some code into Velocity macros to make them usable into Velocity scripts.  The macros will allow to display list of users and lists of groups as they are already displayed by `displayer_users.vm` and `displayer_groups.vm`.
I only mentioned the usage of ``<div>`` (and updating the CSS) as an alternative. The solution I prefer, as I mentioned, is:

> to have a single macro that accepts both a single user (string or document reference) and an array of users. It will always generate a ``<ul>`` even if there is just one user.

So:

    #displayUser('mflorea')
    #displayUser(['mflorea', 'woshilapin'])

and probably also (in the future):

    #displayUser($xcontext.userReference)
    #displayUser([$aliceReference, $bobReference])

Same with ``#displayGroup``. (I'm not fond of 'Profile' because the "user profile" is perceived by our users as the entire ``/xwiki/bin/view/XWiki/mflorea`` page. I'm fine with displayUsers/displayGroups, at plural, though I prefer the singular form)