message: Use ember-data-relationship-tracker in cards editor
diff: import Component from '@ember/component';
  import { inject as service } from '@ember/service';
  +import { get, set } from '@ember/object';
  import layout from '../../templates/components/field-editors/cardstack-cards-editor';
  @@ -9,15 +10,32 @@ export default Component.extend({
  actions: {
  addCard() {
  + let field = get(this, 'field');
  + let content = get(this, 'content');
  +
  this.tools.pickCard().then((card) => {
  - this.get(`content.${this.get('field')}`).pushObject(card);
  + content.watchRelationship(field, () => {
  + get(content, field).pushObject(card);
  + });
  })
  },
  +
  orderChanged(rearrangedCards) {
  - this.set(`content.${this.get('field')}`, rearrangedCards);
  + let field = get(this, 'field');
  + let content = get(this, 'content');
  +
  + content.watchRelationship(field, () => {
  + set(content, field, rearrangedCards);
  + });
  },
  +
  deleteCard(card) {
  - this.get(`content.${this.get('field')}`).removeObject(card);
  + let field = get(this, 'field');
  + let content = get(this, 'content');
  +
  + content.watchRelationship(field, () => {
  + get(content, field).removeObject(card);
  + });
  }
  }
  });
  