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`.
You cannot really use this macro on its own because it's not well isolated:

* it generates a ``<li>`` so you must not forget to wrap the macro call in a ``<ul>`` (which is not obvious unless you read the macro code). Moreover, is the style correctly applied without the ``<ul class="users">``?
* it doesn't import the required CSS so you have to do the ``ssfx.use`` by yourself (but first you must know what to import)

Also, if you want to make the code from the user/group displayer really re-usable it should accept user/group references also (i.e. document references) and not just user alias (as string).

One option 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.

The alternative is to isolate the macro better:
* generate a ``<div>`` instead of ``<li>`` and let ``displayGroupsProfiles`` take care of the ``<ul><li>`` (but you may need to update the CSS which can be tricky)
* do the ``ssfx.use`` here instead of above in ``displayGroupsProfiles``

I prefer the first option.