Use model meta to generate verbose name

Render field names of object rather than the word "None"

This is fix for issue #53, since an object may not exist when rendering
`templates/admin/change_data.html` - ie, if we're greating a new object
when `confirm_add = True` is set on a `ModelAdmin` with
the `AdminConfirmMixin`.

Also makes sure we send an actual object (not the string "None") to the
base template, to remove other instances of the word "None" from
the add view.

admin_confirm/admin.py

@@ -423,13 +423,12 @@ def _change_confirmation_view(self, request, object_id, form_url, extra_context)
             **self.admin_site.each_context(request),
             "preserved_filters": self.get_preserved_filters(request),
             "title": f"{_('Confirm')} {title_action} {opts.verbose_name}",
-            "subtitle": str(obj),
-            "object_name": str(obj),
+            "subtitle": obj and str(obj),
+            "object_name": obj and str(obj),
             "object_id": object_id,
             "app_label": opts.app_label,
             "model_name": opts.model_name,
             "opts": opts,
-            "obj": obj,
             "changed_data": changed_data,
             "add": add,
             "save_as_new": SAVE_AS_NEW in request.POST,

admin_confirm/templates/admin/change_data.html

@@ -9,7 +9,7 @@
     </tr>
     {% for field, values in changed_data.items %}
     <tr>
-      <td>{% verbose_name obj field %}</td>
+      <td>{% verbose_name opts field %}</td>
       <td>{{ values.0|format_change_data_field_value }}</td>
       <td>{{ values.1|format_change_data_field_value }}</td>
     </tr>

admin_confirm/templatetags/formatting.py

@@ -20,6 +20,5 @@ def format_change_data_field_value(field_value):
 
 
 @register.simple_tag
-def verbose_name(obj, fieldname):
-    if obj:
-        return obj._meta.get_field(fieldname).verbose_name
+def verbose_name(opts, fieldname):
+    return opts.get_field(fieldname).verbose_name.capitalize()