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`.
No, indeed, this macro cannot be used alone and that was not the point.  I just made this macro to split the code in simpler functions.  But indeed, maybe any macro defined in this file should be a macro usable on its own.  By the way, the same comment applies for the modifications I proposed for Colibri skin (see other files modified in this pull request).

This pull request is not about CSS modifications.  In fact, it's just a refectoring of `displayer_users.vm` and `displayer_groups.vm` in usable macros for the Colibri skin.  This is why I took the time for it because there was no further investigations to do (just need to reuse existing code and wrap it in a macro).  After that, I just copy/paste these macros in Flamingo skin.  So at this point, I have no idea and not really time to investigate thoroughly CSS stuff.

Anyway, the `<div>` solutions seems to be a good option.  If people are OK to help me on this, I would be happy to report these improvements in this pull request.  For example, if you have an idea on HTML code of the `<div>` with existing classes to use in skins (Flamingo and Colibri), I'll be happy to use them and refactor what I did.

So there is 2 possible action plans here:
* either, we try to improve both macros with `<div>` and CSS; this imply a work on Flamingo AND Colibri versions since pull request propose these macros for the both.
* or we leave it as it is and document that the first macro is not usable on its own (or merge everything in `displayGroupsProfiles` macro to avoid confusion); because I agree that if any macro in this file should be usable on its own, the solution I proposed is misleading.

What do you think?