message: Re-style challenge selection button bar
diff: <div class="form-group">
  <label translate="LABEL_DIFFICULTY"></label>
  <div class="btn-group">
  - <label ng-repeat="difficulty in [1, 2, 3, 4, 5, 6]" class="btn btn-info" ng-model="scoreBoardTablesExpanded[difficulty]" ng-click="toggleDifficulty()" uib-btn-checkbox>
  - <span uib-rating ng-model="difficulty" max="{{difficulty}}" read-only="true" class="nowrap"></span>
  - <span class="badge"><i class="fas fa-check" aria-hidden="true"></i>{{(challenges|filter: {difficulty: difficulty, solved: true}).length + '/' + (challenges|filter: {difficulty: difficulty}).length}}</span>
  + <label ng-repeat="difficulty in [1, 2, 3, 4, 5, 6]" class="btn btn-primary" ng-model="scoreBoardTablesExpanded[difficulty]" ng-click="toggleDifficulty()" uib-btn-checkbox>
  + <span class="fa-4x fa-layers fa-fw">
  + <i class="far fa-star"></i>
  + <span class="fa-layers-text fa-inverse" data-fa-transform="shrink-9" style="font-weight:900;">{{difficulty}}</span>
  + <span ng-class="'fa-layers-counter bg-' + ((challenges|filter: {difficulty: difficulty, solved: true}).length === (challenges|filter: {difficulty: difficulty}).length ? 'success' : (challenges|filter: {difficulty: difficulty, solved: true}).length === 0 ? 'danger' : 'warning')">
  + {{(challenges|filter: {difficulty: difficulty, solved: true}).length + '/' + (challenges|filter: {difficulty: difficulty}).length}}
  + </span>
  + </span>
  </label>
  </div>
  </div>
  