RichTextPage admin needs clean_slug method from PageAdminForm

When saving from the admin interface, the PageAdmin calls save_model, which checks _old_slug attribute. But that attribute is added to the form in clean_slug method declared in PageAdminForm. 

See: https://github.com/stephenmcd/mezzanine/blob/3.0.6/mezzanine/pages/admin.py#L144

Regards,
Ale.