IMPORT_EXPORT_SKIP_ADMIN_EXPORT_UI is not respected

[btimby]

My understanding is that if I set IMPORT_EXPORT_SKIP_ADMIN_EXPORT_UI=True in settings.py then when clicking the export button from change_list will skip the confirmation page and start the export immediately. I wanted to enable this due to #121 since I don't really require this page and it is currently broken.

However, this setting seems not to work. Also, setting ModelAdmin.skip_export_form = True does not work. Trying either of these results in the confirmation page being rendered.