message: Removed enroll button on courses which the Stundent is enrolled
diff: </p>
  </md-card-content>
  <md-card-actions>
  - <button md-icon-button *ngIf="this.userService.isStudent()"
  - disabled={{isMemberOfCourse(course)}}
  + <button md-icon-button *ngIf="this.userService.isStudent() && !this.isMemberOfCourse(course)"
  (click)="apply(course._id, course.hasAccessKey)" mdTooltip="Enroll">
  <md-icon>add_circle_outline</md-icon>
  </button>
  