message: fix: save subjects to relationship endpoint
  When setting the full list of subjects for a registration, use
  `PUT /v2/registrations/<guid>/relationships/subjects/` instead of
  `PATCH /v2/registrations/<guid>/`.
  The latter does not allow setting an empty list.
diff: @@ -57,17 +57,22 @@ export default class SubjectManagerComponent extends Component.extend({
  }).on('init'),
  saveChanges: task(function *(this: SubjectManagerComponent) {
  - this.model.setProperties({
  - subjects: this.selectedSubjects,
  - });
  + const { selectedSubjects } = this;
  try {
  - yield this.model.save();
  + yield this.model.updateM2MRelationship('subjects', selectedSubjects);
  } catch (e) {
  this.toast.error(this.i18n.t('registries.registration_metadata.save_subjects_error'));
  throw e;
  }
  - yield this.initializeSubjects.perform();
  +
  + const savedSubjectIds = new Set(selectedSubjects.map(s => s.id));
  + this.setProperties({
  + savedSubjectIds,
  + selectedSubjectIds: new Set(savedSubjectIds),
  + });
  + this.incrementProperty('selectedSubjectsChanges');
  + this.incrementProperty('savedSubjectsChanges');
  }).drop(),
  }) {
  // required
  