Added unsplash button/selector to publication cover image form field

no issue

- shows an unsplash button when no image is present, opening the full unsplash selector when clicked
- behind the `improvedOnboarding` labs flag

ghost/admin/app/components/settings/form-fields/publication-cover.hbs

@@ -29,9 +29,16 @@
                     </button>
                 </div>
             {{else}}
-                <button id="publication-cover" type="button" class="gh-btn gh-btn-white self-start" {{on "click" uploader.triggerFileDialog}} data-test-image-upload-btn="coverImage">
-                    <span>Upload cover</span>
-                </button>
+                <div class="self-start">
+                    <button id="publication-cover" type="button" class="gh-btn gh-btn-white" {{on "click" uploader.triggerFileDialog}} data-test-image-upload-btn="coverImage">
+                        <span>Upload cover</span>
+                    </button>
+                    {{#if (feature "improvedOnboarding")}}
+                        <button type="button" class="gh-btn gh-btn-white w10" {{on "click" this.toggleUnsplashSelector}} aria-label="Select photo from Unsplash">
+                            <span>{{svg-jar "unsplash"}}</span>
+                        </button>
+                    {{/if}}
+                </div>
             {{/if}}
             <div class="dn">
                 <GhFileInput
@@ -45,4 +52,11 @@
         </div>
     </div>
     </GhUploader>
+
+    {{#if this.showUnsplashSelector}}
+        <GhUnsplash
+            @select={{this.setUnsplashImage}}
+            @close={{this.toggleUnsplashSelector}}
+        />
+    {{/if}}
 </div>

ghost/admin/app/components/settings/form-fields/publication-cover.js

@@ -5,10 +5,14 @@ import {
 } from 'ghost-admin/components/gh-image-uploader';
 import {action} from '@ember/object';
 import {inject as service} from '@ember/service';
+import {tracked} from '@glimmer/tracking';
 
 export default class PublicationCoverFormField extends Component {
+    @service feature;
     @service settings;
 
+    @tracked showUnsplashSelector = false;
+
     imageExtensions = IMAGE_EXTENSIONS;
     imageMimeTypes = IMAGE_MIME_TYPES;
 
@@ -24,4 +28,16 @@ export default class PublicationCoverFormField extends Component {
         this.settings.set('coverImage', value);
         this.args.didUpdate('coverImage', value);
     }
+
+    @action
+    toggleUnsplashSelector() {
+        if (this.feature.improvedOnboarding) {
+            this.showUnsplashSelector = !this.showUnsplashSelector;
+        }
+    }
+
+    @action
+    setUnsplashImage({src}) {
+        this.update(src);
+    }
 }