Improve the way the group page looks like

Example: XWiki.XWikiAllGroup

This issue is related to XWIKI-1901. If we were to display the list of groups, then we should improve how a group page looks like.

• while XWiki.XWikiAllGroup is marked hidden by default, the new created groups are not - we need to make a rule about this (related to the displaying in the XWiki space)

• while user profile pages are not childs of User Directory page
  'Wiki Home » User Directory » Profile of Caty' the groups have this hierarchy 'Wiki Home » XWiki Space » Data types » XWiki Group Class » FrontEnd'

• The group page could be just a simple page listing the users, but we could:

• add the ability to manage the users from the group page (add, edit, delete from group functionality);
• display the activity stream for the specific group;
• be able to define custom columns for the group page (just like we have for the user directory), depending on the group highlighting e-mail address or speciality of the user, etc.
• functionality to create/view the wiki/workspace for the group;
• etc.