Fix bug 826444: Disable "fullscreen" section editing.

Since we've decided the only way we want to support section editing is
through the inline AJAX-y edit, this disables the ability to do it
through any other kind of request, and updates the section-editing
unit tests to match that.

I am more than open to a better thing to do when getting a non-AJAX request; right now it just spits out a (non-translated) "you can't do that" message.