message: Improve layout of toggle buttons for difficulties
diff: <uib-progressbar animate="false" value="percentChallengesSolved" type="{{completionColor}}"><b>{{percentChallengesSolved}}%</b></uib-progressbar>
  + <div class="container-fluid well">
  + <div class="form-group">
  + <label translate="LABEL_DIFFICULTY"></label>
  <div class="btn-group">
  - <div ng-repeat="difficulty in [1, 2, 3, 4, 5]">
  - <button type="button" class="btn btn-default" ng-click="toggleCollapsed(difficulty)"><span uib-rating ng-model="difficulty" max="{{difficulty}}" read-only="true" class="nowrap"></span></button>
  + <label ng-repeat="difficulty in [1, 2, 3, 4, 5]" class="btn btn-info" ng-model="!scoreBoardTablesCollapsed[difficulty]" ng-click="toggleCollapsed(difficulty)" uib-btn-checkbox><span uib-rating ng-model="difficulty" max="{{difficulty}}" read-only="true" class="nowrap"></span></label>
  + </div>
  </div>
  </div>
  - <hr>
  +
  <div ng-repeat="difficulty in [1, 2, 3, 4, 5]">
  - <section uib-collapse="scoreBoardTablesCollapsed[difficulty]" class="well well-lg">
  + <section uib-collapse="scoreBoardTablesCollapsed[difficulty]" class="container-fluid well">
  <h3>
  <span uib-rating ng-model="difficulty" max="{{difficulty}}" read-only="true" class="nowrap"></span>
  </h3>
  