has_review_queue has no access to ReviewedWidgySite

When `widgy.contrib.review_queue` is not in `INSTALLED_APPS` and the demo project uses `widgy.site.WidgySite`, navigating to any widgy page change form in the admin center throws this NameError:

```
Traceback (most recent call last):
  File "/Users/scottclark/.virtualenvs/django-widgy/lib/python2.7/site-packages/django/contrib/staticfiles/handlers.py", line 72, in __call__
    return self.application(environ, start_response)
  File "/Users/scottclark/.virtualenvs/django-widgy/lib/python2.7/site-packages/django/core/handlers/wsgi.py", line 255, in __call__
    response = self.get_response(request)
  File "/Users/scottclark/.virtualenvs/django-widgy/lib/python2.7/site-packages/django/core/handlers/base.py", line 176, in get_response
    response = self.handle_uncaught_exception(request, resolver, sys.exc_info())
  File "/Users/scottclark/.virtualenvs/django-widgy/lib/python2.7/site-packages/django/core/handlers/base.py", line 215, in handle_uncaught_exception
    return debug.technical_500_response(request, *exc_info)
  File "/Users/scottclark/.virtualenvs/django-widgy/lib/python2.7/site-packages/django_extensions/management/technical_response.py", line 5, in null_technical_500_response
    six.reraise(exc_type, exc_value, tb)
  File "/Users/scottclark/.virtualenvs/django-widgy/lib/python2.7/site-packages/django/core/handlers/base.py", line 113, in get_response
    response = callback(request, *callback_args, **callback_kwargs)
  File "/Users/scottclark/.virtualenvs/django-widgy/lib/python2.7/site-packages/django/contrib/admin/options.py", line 392, in wrapper
    return self.admin_site.admin_view(view)(*args, **kwargs)
  File "/Users/scottclark/.virtualenvs/django-widgy/lib/python2.7/site-packages/django/utils/decorators.py", line 91, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File "/Users/scottclark/.virtualenvs/django-widgy/lib/python2.7/site-packages/django/views/decorators/cache.py", line 89, in _wrapped_view_func
    response = view_func(request, *args, **kwargs)
  File "/Users/scottclark/.virtualenvs/django-widgy/lib/python2.7/site-packages/django/contrib/admin/sites.py", line 202, in inner
    return view(request, *args, **kwargs)
  File "/Users/scottclark/.virtualenvs/django-widgy/lib/python2.7/site-packages/mezzanine/pages/admin.py", line 120, in change_view
    return super(PageAdmin, self).change_view(request, object_id, **kwargs)
  File "/Users/scottclark/.virtualenvs/django-widgy/lib/python2.7/site-packages/django/utils/decorators.py", line 25, in _wrapper
    return bound_func(*args, **kwargs)
  File "/Users/scottclark/.virtualenvs/django-widgy/lib/python2.7/site-packages/django/utils/decorators.py", line 91, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File "/Users/scottclark/.virtualenvs/django-widgy/lib/python2.7/site-packages/django/utils/decorators.py", line 21, in bound_func
    return func(self, *args2, **kwargs2)
  File "/Users/scottclark/.virtualenvs/django-widgy/lib/python2.7/site-packages/django/db/transaction.py", line 223, in inner
    return func(*args, **kwargs)
  File "/Users/scottclark/.virtualenvs/django-widgy/lib/python2.7/site-packages/django/contrib/admin/options.py", line 1171, in change_view
    return self.render_change_form(request, context, change=True, obj=obj, form_url=form_url)
  File "/Users/scottclark/www/django-widgy/widgy/contrib/widgy_mezzanine/admin.py", line 163, in render_change_form
    if self.has_review_queue:
  File "/Users/scottclark/www/django-widgy/widgy/contrib/widgy_mezzanine/admin.py", line 175, in has_review_queue
    return isinstance(self.get_site(), ReviewedWidgySite)
NameError: global name 'ReviewedWidgySite' is not defined
```