Only serve statistics publications from /statistics

They were still routable from /publications, as well as /statistics, which is bad.
Serve them from one authoritative place, and support any use of old style with redirects

Removes a couple of old tests for statistics-behaviour on publication views. The first
of which failed when adding the redirect, the 2nd two silently pass, as they're refuting
a selector against an empty body.

https://www.agileplannerapp.com/boards/105200/cards/5540
Worth pulling this out to a before_filter for clarity and so the other stuff gets skipped? e.g.

```ruby
before_filter :redirect_statistics_publications_to_new_url, only: :show

private

def redirect_statistics_publications_to_new_url
  if @document.statistics?
    redirect_to public_document_url(@document), status: :moved_permanently
  end
end
```