add favorite_border icon to comment component

src/app/components/shared/comment/comment.component.html

@@ -6,7 +6,7 @@
         <mat-icon [ngClass]="{true : 'correct-icon', false: 'incorrect-icon'}[comment.correct]">check_circle</mat-icon>
       </button>
       <button mat-icon-button *ngIf="comment.favorite || !isStudent" [disabled]="isStudent" (click)="setFavorite(comment)" [matTooltip]="comment.favorite ? 'Mark as not favorite' : 'Mark as favorite'">
-        <mat-icon [ngClass]="{true: 'favorite-icon', false: 'not-favorite-icon'}[comment.favorite]">star</mat-icon>
+        <mat-icon [ngClass]="{true: 'favorite-icon', false: 'not-favorite-icon'}[comment.favorite]">favorite_border</mat-icon>
       </button>
       <button mat-icon-button [disabled]="isStudent" (click)="setRead(comment)" [matTooltip]="comment.read ? 'Mark  as unread' : 'Mark as read'">
         <mat-icon [ngClass]="{true: 'read-icon', false: 'unread-icon'}[comment.read]">visibility</mat-icon>