Added support for feature-gated editor cards

refs https://github.com/TryGhost/Team/issues/910

- added `card-is-available` helper that handles the previous `card.developerExperiment` and the new `card.feature` options
- updated `<KoenigMenuContent>` to use the `card-is-available` helper in the conditional when adding cards to the list

Cards can be gated by feature when specifying them in `cards.js`, in the card definition object you can add `feature: 'flagName'` which will mean the card is only available when that feature is enabled.

ghost/admin/lib/koenig-editor/addon/components/koenig-menu-content.hbs

@@ -5,7 +5,7 @@
                 {{section.title}}
             </div>
             {{#each section.items as |item|}}
-                {{#if (or (not item.developerExperiment) (enable-developer-experiments))}}
+                {{#if (card-is-available item)}}
                     <div
                         class="{{if (eq item @selectedItem) "kg-cardmenu-card-selected"}} {{kg-style "cardmenu-card"}}"
                         data-kg="cardmenu-card"

ghost/admin/lib/koenig-editor/addon/helpers/card-is-available.js

@@ -0,0 +1,22 @@
+import Helper from '@ember/component/helper';
+import {get} from '@ember/object';
+import {inject as service} from '@ember/service';
+
+export default class CardIsAvailableHelper extends Helper {
+    @service config;
+    @service feature;
+
+    compute([card]) {
+        let cardIsAvailable = true;
+
+        if (card.developerExperiment) {
+            cardIsAvailable = this.config.get('enableDeveloperExperiments');
+        }
+
+        if (card.feature) {
+            cardIsAvailable = get(this.feature, card.feature);
+        }
+
+        return cardIsAvailable;
+    }
+}

ghost/admin/lib/koenig-editor/app/helpers/card-is-available.js

@@ -0,0 +1 @@
+export {default} from 'koenig-editor/helpers/card-is-available';