message: Fix missing lecture wide unit drop target
diff: </div>
  <div class="unit-drop-target"
  - *ngIf="getDataForKey('openLectureId') === lecture._id && !lecture.units.length && isDraggingUnit()"
  + *ngIf="getDataForKey('openLectureId') &&
  + (getDataForKey('openLectureId') !== lecture._id || !lecture.units.length) &&
  + isDraggingUnit()"
  dragula="units"
  [dragulaModel]="lecture.units"
  [attr.data-lecture-id]="lecture._id">
  