message: Makes expandable div to h2
diff: <div class="clickable" (click)="notify()">
  - <h1>{{title}}
  + <h2>
  + {{title}}
  <button class="toggle" [class.opened] = "opened" mat-icon-button>
  <mat-icon>arrow_drop_down</mat-icon>
  </button>
  - </h1>
  + </h2>
  </div>
  