Added developer experiments indication to about page

no issue

- without this there's no indication the config flag is set when you have no access to the config file or you don't know which features are currently behind the flag

ghost/admin/app/helpers/enable-developer-experiments.js

@@ -1,10 +1,10 @@
 import Helper from '@ember/component/helper';
 import {inject as service} from '@ember/service';
 
-export default Helper.extend({
-    config: service(),
+export default class EnableDeveloperExperimentsHelper extends Helper {
+    @service config;
 
     compute() {
         return this.config.get('enableDeveloperExperiments');
     }
-});
+}

ghost/admin/app/templates/about.hbs

@@ -9,6 +9,9 @@
                 <li><strong>Environment</strong> {{this.config.environment}}</li>
                 <li class="gh-env-list-database-type"><strong>Database</strong> {{this.config.database}}</li>
                 <li><strong>Mail</strong> {{#if this.config.mail}}{{this.config.mail}}{{else}}Native{{/if}}</li>
+                {{#if (enable-developer-experiments)}}
+                <li><strong>Developer experiments</strong> Enabled</li>
+                {{/if}}
             </ul>
             <div class="gh-env-help">
                 <a class="gh-btn" href="https://ghost.org/docs/" target="_blank"><span>User Documentation</span></a>