message: add lock and unlock icon to course overview for identifying the access type of the course
diff: </button>
  </span>
  <span class="course-student-actions" *ngIf="this.userService.isStudent()">
  + <div style="float:left">
  + <i class="material-icons" style="margin-top: 8px;"
  + matTooltip="course locked by access key"
  + *ngIf="course.enrollType == 'accesskey'">lock</i>
  + <i class="material-icons" style="margin-top: 8px;"
  + matTooltip="free course"
  + *ngIf="course.enrollType != 'accesskey'">lock_open</i>
  + </div>
  + <div style="float:left">
  <button mat-icon-button *ngIf="!this.isMemberOfCourse(course)"
  (click)="enroll()" matTooltip="Enroll">
  <mat-icon>add_circle_outline</mat-icon>
  </button>
  + </div>
  <button mat-icon-button *ngIf="this.isMemberOfCourse(course)"
  (click)="leave()" matTooltip="Leave">
  