Show stats nav and remove beta notices

A tabbed navigation allowing users to navigate to the
stats announcements index from the statistics page was
commented out as a feature-flag.

This reinstates it and removes the beta notices on the
stats announcements pages.

Also renames a couple of mis-named scss modules.
/government/statistics shows filtered index of statistics

Statistics are being extracted from the publications section of the site
to be displayed under /government/statistics. This commit creates a new
statistics controller.

It's more of a smoke-and-mirrors approach; The new StatisticsController
inherits from PublicationsController and gets most of it's functionality
from there.

Some thought was given into whether building a new provider in a similar
pattern to the stats announcements, but we decided that since gds hasn't
decided on what shape this kind of thing should be yet, it would be better
to take a less drastic approach for now.
Statistics show pages live at /gov/statistics/:slug

There wasn't much to do here as the new StatisticsController
already inherited everything it needed to show statistics pubs
from the publications controller.
Add impartiality notice to statistics views
Add restyled beta notice to statistics and stats announcements
Statistics controller inherits from documents controller

Redirects non XHR requests for statistics from publications controller
to statistics index.
Show statistics notice when filtering documents.

Adds a custom message notifying the user of the new statistics section
when filtering publications on statistics document type.
Reorganise beta notices
Scope beta and impartiality styles
Shortcut ajax call when statistics are selected
Remove trailing whitespace
Publications#index retains params when redirecting to stats#index
Merge pull request #1609 from alphagov/nav-to-stats-announcements

Move published and upcoming statistics to their own section