message: Added default text for empty courses
diff: <h2>{{course.name}}</h2>
  +<ng-container *ngIf="!course.lectures.length">No lectures to download!</ng-container>
  <mat-list *ngFor=" let lecture of course.lectures">
  <mat-list-item class="lecture">{{lecture.name}}</mat-list-item>
  <mat-list *ngFor=" let unit of lecture.units">
  