Add "power users" group as functional administrators

Introduce a new group "powerusers" so that users member of this group can manage in the Admin Center:

• vocabularies management
• users and groups management
• default dashboard management

Add new filters:

• usersGroupsManagementAccess to allow management of Users and Groups
• directoriesManagementAccess to allow management of Directories (Vocabularies)
• dashboardManagementAccess to allow management of default dashboard
• socialCollaborationManagementAccess to allow management of Social Collaboration

Check for those filters in the templates / Seam beans where we were checking for NuxeoPrincipal#isAdministrator().