closes https://github.com/TryGhost/Ghost/issues/9840
- added error on uploading an invalid image format
- updated to use overall error method(`onFailed`) to set error message instead of individual upload failures(`onUploadFailure`)
no issue
- added `onUploadStart` action to `{{gh-uploader}}` that is triggered when individual files start uploading
- will not be triggered for any files that fail client-side validation
- changed `{{koenig-card-gallery}}` to only add images to the local gallery display when the uploader starts an upload
no issue
- moved all payload building into single function
- only put fully valid images into mobiledoc
- extracted image data reading and width/height reading into a separate function
- fixed re-ordering when deleting an image