message: Remove disabled property from unavailable challenges
  (Does not display tooltip on button in that case.)
diff: </button>
  <button *ngIf="challenge.disabledEnv" [id]="challenge.name + '.unavailable'" mat-raised-button
  - [matTooltip]="challenge.hint" matTooltipPosition="above" disabled>
  + [matTooltip]="challenge.hint" matTooltipPosition="above">
  <span>
  <i class="{{ 'icon-' + challenge.disabledEnv?.toString().toLowerCase() }}"></i>
  {{'STATUS_UNAVAILABLE' | translate}}
  