🐛 Fixed pasted image files appearing twice in editor

closes https://github.com/TryGhost/Team/issues/1254

- when switching over to a more generic file paste handling the prevention of the default paste behaviour was missed meaning an additional image element was added to the contenteditable area which was not rendered on the front-end and disappeared when reloading the post

lib/koenig-editor/addon/components/koenig-editor.js

@@ -966,6 +966,9 @@ export default Component.extend({
             editor.run((postEditor) => {
                 insertCardsFromFiles(event.clipboardData.files, postEditor);
             });
+            // prevent mobiledoc's default paste event handler firing
+            event.preventDefault();
+            event.stopImmediatePropagation();
             return;
         }