Only serve statistics publications from /statistics

They were still routable from /publications, as well as /statistics, which is bad.
Serve them from one authorative 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 addin the redirect, the 2nd 2 silently pass, as they're refuting
a selector against an empty body.
Extract logic to before_filter for clarity

Statistics are now served under their own routing (i.e. under
/government/statistics/), so we should redirect any requests at
/government/publications/ there when the publication is a statistics
document.
Data migration to cleanup search index

This will remove the search index entries for the old-style publication
paths and re-index them with their new path.
Reinstate test coverage on national stats badge
Be more explicit about where the redirect is going

This makes it clearer that the redirect is sending the request to to a
different controller
Merge pull request #1672 from alphagov/duplicate-statistics-urls

Only serve statistics publications from /statistics