Pass image_id into confirmation.json (as it was before)

This is because when the image is deleted, the ID gets unassigned. So "None" gets inserted into the response JSON.

wagtail/wagtailimages/templates/wagtailimages/multiple/confirmation.json

@@ -1,5 +1,5 @@
 {
-    "image_id": {{ image.id }},
+    "image_id": {{ image_id }},
 {% if form %}
     "form": "{% filter escapejs %}{% include 'wagtailimages/multiple/edit_form.html' %}{% endfilter %}",
 {% endif %}

wagtail/wagtailimages/views/multiple.py

@@ -56,11 +56,12 @@ def edit(request, image_id, callback=None):
         form.save()
         return render(request, 'wagtailimages/multiple/confirmation.json', {
             'success': True,
-            'image': image,
+            'image_id': image_id,
         }, content_type='application/json')
     else:
         return render(request, 'wagtailimages/multiple/confirmation.json', {
             'success': False,
+            'image_id': image_id,
             'image': image,
             'form': form,
         }, content_type='application/json')
@@ -81,6 +82,6 @@ def delete(request, image_id):
 
     return render(request, 'wagtailimages/multiple/confirmation.json', {
         'success': True,
-        'image': image,
+        'image_id': image_id,
     }, content_type='application/json')