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
I've just taken a look at the test coverage for the statistics controller show action. There's one feature covering it, but no functional tests. I think these tests want to be moved there instead of being removed, and maybe some of the other tests here would be useful coverage.