Added basic UI for collection posts

refs https://github.com/TryGhost/Team/issues/3169

- Added a lits to a collection view showing which posts belong to a collection.

ghost/admin/app/models/collection.js

@@ -1,5 +1,7 @@
-import Model, {attr} from '@ember-data/model';
+import Model from '@ember-data/model';
 import ValidationEngine from 'ghost-admin/mixins/validation-engine';
+import {attr} from '@ember-data/model';
+import {computed} from '@ember/object';
 import {inject as service} from '@ember/service';
 
 export default Model.extend(ValidationEngine, {
@@ -17,5 +19,15 @@ export default Model.extend(ValidationEngine, {
     updatedBy: attr('number'),
     count: attr('raw'),
 
+    posts: attr('raw'),
+
+    postIds: computed('posts', function () {
+        if (this.posts && this.posts.length) {
+            return this.posts.map(post => post.id);
+        } else {
+            return [];
+        }
+    }),
+
     feature: service()
 });

ghost/admin/app/templates/collection.hbs

@@ -34,4 +34,15 @@
             </button>
         </div>
     {{/unless}}
+
+    {{#if this.collection.postIds}}
+    <div class="gh-main-section">
+        <h3>Collection has {{this.collection.postIds.length}} posts</h3>
+        <ol class="gh-list">
+            {{#each this.collection.postIds as |post|}}
+            <li class="gh-list-row"><a href="#/editor/post/{{post}}">{{post}}</a></li>
+            {{/each}}
+        </ol>
+    </div>
+    {{/if}}
 </section>