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
Remove international development fund categories

International development funds are moving from whitehall to
specialist-publisher/specialist-frontend. Therefore we want to remove
them from collections browse and redirect the categories to the new
international development fund finder.
Nullify the correct mainstream category field

Sometimes the category we're removing isn't the primary category but one
of the other categories. Handle that case.
Merge pull request #1672 from alphagov/duplicate-statistics-urls

Only serve statistics publications from /statistics
Update dashboard link destinations

* Include ‘www’ in support link
* Switch development backlog from pivotal to agile planner
Merge pull request #1700 from alphagov/fix-whitehall-dashboard-links

Update dashboard link destinations
Notify the router API to add redirects
Merge pull request #1698 from alphagov/remove-international-development-fund-categories

Remove international development fund categories