[AngularJS] Extract predicate methods into filters for ng-if and ng-show

Leaking logic in controllers is not an option, filters are a way to refactor your code and are compatible with ng-if and ng-show.

    <div ng-if="main.currentUser | user:'isAdmin'">
      Admin div
    </div>
    <div ng-if="main.currentUser | user:'isntAdmin'">
      Standard user div
    </div>

var app = angular.module('App', []);

app.controller('MainCtrl', function() {
  var currentUser = { rights: [] };
  function setAdmin(){
    resetAdmin();
    currentUser.rights.push('admin');
  }
  function resetAdmin(){
    currentUser.rights = [];
  }

  this.currentUser = currentUser;
  this.setAdmin    = setAdmin;
  this.resetAdmin  = resetAdmin;
});

app.filter('user', function(){
  var rules = {
    isAdmin: function(user){
      return user.rights.indexOf('admin') !== -1;
    },
    isntAdmin: function(user){
      return !rules.isAdmin(user);
    }
  };

  return function(user, rule){
    return rules[rule](user);
  };
});