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
This migration looks the right shape, but I can't figure out what generates the paths involved.  In particular, I can't find any references to the name publication_path elsewhere in the whitehall code (apart from in tests).

This is probably me not knowing enough about whitehall, but I'd love a pointer to where publication_path is defined.