[FIX] website: remove "add image" button from the normal mode

The "Add image" button was shown out of the editor mode when removing
all images from a snippet.

task-2469516

closes odoo/odoo#71486

X-original-commit: 3bae864d1e587e87262c56f1c5f789ac6a6fabc4
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/website/static/src/snippets/s_image_gallery/options.js

@@ -239,7 +239,7 @@ options.registry.gallery = options.Class.extend({
      */
     removeAllImages: function (previewMode) {
         var $addImg = $('<div>', {
-            class: 'alert alert-info css_editable_mode_display text-center',
+            class: 'alert alert-info css_non_editable_mode_hidden text-center',
         });
         var $text = $('<span>', {
             class: 'o_add_images',