message: Added indicator for not visible unit
diff: <div *ngFor="let unit of units;">
  <mat-card class="unit-card">
  + <button mat-icon-button *ngIf="unit.visible === false" disabled>
  + <mat-icon color="warn">visibility_off</mat-icon>
  + </button>
  <mat-card-title class="unit-card-title">
  {{unit.name}}
  </mat-card-title>
  <app-code-kata *ngSwitchCase="'code-kata'" [codeKataUnit]="unit"></app-code-kata>
  <app-free-text-unit *ngSwitchCase="'free-text'" [freeTextUnit]="unit"></app-free-text-unit>
  <div class="unit-detail">
  - <div class="user-detail" *ngIf="unit.unitCreator">{{unit.unitCreator.profile.firstName + ' ' + unit.unitCreator.profile.lastName}}</div>
  + <div class="user-detail" *ngIf="unit.unitCreator">{{unit.unitCreator.profile.firstName + ' ' +
  + unit.unitCreator.profile.lastName}}
  + </div>
  <span *ngIf="unit.createdAt === unit.updatedAt">Published on: {{getFormattedDate(unit.createdAt)}}</span>
  <span *ngIf="unit.createdAt !== unit.updatedAt">Changed on: {{getFormattedDate(unit.updatedAt)}}</span>
  </div>
  