message: Fxied text in download
diff: <h2>{{course.name}}</h2>
  -<ng-container *ngIf="!this.checkForEmptyLectures()">No lectures to download!</ng-container>
  +<ng-container *ngIf="!this.checkForEmptyLectures()">No course material available for download</ng-container>
  <div *ngIf="this.checkForEmptyLectures()">
  -<p>Select units or files which you want to download</p>
  +<p>Select units or files which you want to download in a zip</p>
  <mat-dialog-content class="layout">
  <mat-list>
  