show warning banner for "private" blogs

[snarfed]

wordpress.com: http://en.support.wordpress.com/settings/privacy-settings/ https://developer.wordpress.com/docs/api/1/get/sites/%24site/#apidoc-response

blogger: https://support.google.com/blogger/answer/42673

tumblr: ???

[snarfed]

deprioritizing, since the current webmention endpoint check also covers this.

[snarfed]

first user in the wild hit this: https://www.brid.gy/blogger/1859345614386780255 , http://myyearphotos.blogspot.com/

[snarfed]

should be pretty easy to just show an extra banner, like we do when they haven't added the webmention endpoint yet.