message: fix: unselecting unseen child subjects
  When checking for child subjects that should be implicitly removed,
  check their parent ids instead of comparing `subject === child.parent`
  directly.
diff: @@ -174,7 +174,7 @@ export default class SubjectManagerComponent extends Component.extend({
  if (this.subjectHasSelectedChildren(subject)) {
  this.selectedSubjects
  - .filterBy('parent', subject)
  + .filter(s => s.belongsTo('parent').id() === subject.id)
  .forEach(s => this.unselectSubject(s));
  }
  }
  