Fix #501

 - Added the functionality "disabling an user", only for the admin user. 
 - You can disable the user from the Account > Users Menu , only in 'Friends' and 'Search' tabs.
 - A special css style has been implemented for a desactivated user ( we may need to change the color, I have a pretty bad taste for that kind of things :).
 - The following cache are flushed when you activate / desactivate an user : 
  - group-cache
  - group-user-cache
  - suggest-users-cache
 - The SearchBar ( header ) can retrieve desactivated users but apply a specific css style to inform the current user that the searched user is desactivated.
 - The group counter doesn't count desactivated users but show them in the members list.
 - The Js Models & views have been updated with two new parameters :
  - 'activated'  : field's value in cassandra
  - 'desactivable' : show the desactivate button or not. Default is false.