Fixed an error on the KoenigBasicHtmlTextarea

refs https://github.com/TryGhost/Team/issues/1206

- Backspace was triggering the error due to undefined `SPECIAL_MARKUPS`

ghost/admin/lib/koenig-editor/addon/components/koenig-basic-html-textarea.js

@@ -110,6 +110,8 @@ export default Component.extend({
 
     init() {
         this._super(...arguments);
+
+        this.SPECIAL_MARKUPS = SPECIAL_MARKUPS;
     },
 
     didReceiveAttrs() {