Admin lets editors try to attach blog posts to places that blog posts don't belong.

[rjsparks]

This results in 500s until the page is rehomed. Example:

[Django] ERROR (EXTERNAL IP): Internal Server Error: /blog/ietf116-new-topics/next-steps-towards-a-net-zero-ietf/

The editing page should object if a blog page is placed anywhere other than /blog.