add webmentions feature flag

refs #2406
Adding the alpha feature flag so we can begin testing.

ghost/admin/app/services/feature.js

@@ -64,6 +64,7 @@ export default class FeatureService extends Service {
     @feature('audienceFeedback') audienceFeedback;
     @feature('suppressionList') suppressionList;
     @feature('emailStability') emailStability;
+    @feature('webmentions') webmentions;
 
     _user = null;
 

ghost/admin/app/templates/settings/labs.hbs

@@ -226,6 +226,19 @@
                         </div>
                     </div>
                 </div>
+                <div class="gh-expandable-block">
+                    <div class="gh-expandable-header">
+                        <div>
+                            <h4 class="gh-expandable-title">Webmentions</h4>
+                            <p class="gh-expandable-description">
+                                Allow receiving webmentions from external sites and sending webmentions when linking external sites in posts.
+                            </p>
+                        </div>
+                        <div class="for-switch">
+                            <GhFeatureFlag @flag="webmentions" />
+                        </div>
+                    </div>
+                </div>
             </div>
         </div>
         {{/if}}

ghost/core/core/shared/labs.js

@@ -32,7 +32,8 @@ const BETA_FEATURES = [
 const ALPHA_FEATURES = [
     'urlCache',
     'beforeAfterCard',
-    'lexicalEditor'
+    'lexicalEditor',
+    'webmentions'
 ];
 
 module.exports.GA_KEYS = [...GA_FEATURES];