Optimised gallery image upload previews (#1079)

no issue
- use `URL.createObjectURL(file)` to get a blob url rather than using `FileReader.readAsDataURL` which generates a very large data attribute
- speeds up the display of previews and associated browser hangs when an upload finishes, especially noticeable with large images and fast connections where multiple uploads finish around the same time

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

@@ -228,30 +228,22 @@ export default Component.extend({
     },
 
     _readDataFromImageFile(file) {
-        let reader = new FileReader();
+        let url = URL.createObjectURL(file);
         let image = EmberObject.create({
-            fileName: file.name
+            fileName: file.name,
+            previewSrc: url
         });
 
-        reader.onload = (e) => {
-            let imgElement = new Image();
-            let previewSrc = htmlSafe(e.target.result);
+        let imageElem = new Image();
+        imageElem.onload = () => {
+            // update current display images
+            image.set('width', imageElem.naturalWidth);
+            image.set('height', imageElem.naturalHeight);
 
-            image.set('previewSrc', previewSrc);
-
-            imgElement.onload = () => {
-                // update current display images
-                image.set('width', imgElement.width);
-                image.set('height', imgElement.height);
-
-                // ensure width/height makes it into the payload images
-                this._buildAndSaveImagesPayload();
-            };
-
-            imgElement.src = previewSrc;
+            // ensure width/height makes it into the payload images
+            this._buildAndSaveImagesPayload();
         };
-
-        reader.readAsDataURL(file);
+        imageElem.src = url;
 
         return image;
     },