Added milestone emails alpha flag

refs https://www.notion.so/ghost/Marketing-Milestone-email-campaigns-1d2c9dee3cfa4029863edb16092ad5c4

- Added an alpha feature flag toggle for milestone emails

ghost/admin/app/services/feature.js

@@ -71,6 +71,7 @@ export default class FeatureService extends Service {
     @feature('outboundLinkTagging') outboundLinkTagging;
     @feature('webmentionEmail') webmentionEmail;
     @feature('emailErrors') emailErrors;
+    @feature('milestoneEmails') milestoneEmails;
 
     _user = null;
 

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

@@ -265,6 +265,19 @@
                         </div>
                     </div>
                 </div>
+                <div class="gh-expandable-block">
+                    <div class="gh-expandable-header">
+                        <div>
+                            <h4 class="gh-expandable-title">Milestone emails</h4>
+                            <p class="gh-expandable-description">
+                                Send emails for reaching specific milestones.
+                            </p>
+                        </div>
+                        <div class="for-switch">
+                            <GhFeatureFlag @flag="milestoneEmails" />
+                        </div>
+                    </div>
+                </div>
             </div>
         </div>
         {{/if}}

ghost/core/core/shared/labs.js

@@ -36,7 +36,8 @@ const ALPHA_FEATURES = [
     'lexicalEditor',
     'webmentions',
     'webmentionEmail',
-    'outboundLinkTagging'
+    'outboundLinkTagging',
+    'milestoneEmails'
 ];
 
 module.exports.GA_KEYS = [...GA_FEATURES];