Fixes a number of issues with the image uploader. Read below,

- Added a remove link to remove files that have an error.
- Error will appear below the progress bar.
- Hovering on dz-image or dz-details will display the error message. Otherwise error message was covering the remove link as well.
- Removed styling around the file size.
- Removed gradient effect in accordance with BookStack styling.
- Dropzone filenae will not overflow the container element. Also done for page attachments
- Added a 'uploaded' error message. this error was being thrown when the file size exceeded the server configured file size. (https://stackoverflow.com/a/42934387/903324)

Towards #741

Signed-off-by: Abijeet <abijeetpatro@gmail.com>

resources/assets/js/vues/components/dropzone.js

@@ -13,6 +13,8 @@ function mounted() {
    let container = this.$el;
    let _this = this;
    new DropZone(container, {
+	addRemoveLinks: true,
+	dictRemoveFile: trans('components.image_upload_remove'),
         url: function() {
             return _this.uploadUrl;
         },
@@ -41,7 +43,7 @@ function mounted() {
                     $(file.previewElement).find('[data-dz-errormessage]').text(message);
                 }
 
-                if (xhr.status === 413) setMessage(trans('errors.server_upload_limit'));
+                if (xhr && xhr.status === 413) setMessage(trans('errors.server_upload_limit'));
                 if (errorMessage.file) setMessage(errorMessage.file[0]);
             });
         }
@@ -57,4 +59,4 @@ module.exports = {
     props,
     mounted,
     data,
-};
+};