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.
Don't overwrite default behaviour of span2 class

* Styles made all span2 classes expand to 100% at small widths
* Instead, do this explicitly with a new class

(Bootstrap 3 has proper hooks for doing this)
Remove unused nav-header overrides

* There are no nav-headers being used in the admin app anywhere
Simplify masthead by not basing it upon bootstrap

* The primary navigation of Whitehall is nothing like a Bootstrap
navbar. If we don’t have to override all those Bootstrap styles the CSS
can be simplified a lot.
* Use the `masthead` class, and split into two sections, details and
tabs.
* Simplify markup by removing nested containers
* Make visuallyhidden a re-usable class by taking it out of the nav
specific styles
* Keep the header looking exactly as it did before (excluding the
horrible text shadows)
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
Further simplify masthead styling

* Use tab and menu item specific classes for clearer selector intent
* Make ‘All users’ and ‘Import’ links a standard link_to, on preview
there’s a display bug when these links are active
* Be clearer about when a link is for a menu or a tab
Use appropriate IDs for javascript links

* Each drop down menu link should point to the ID of the element it
displays (see
https://github.com/alphagov/whitehall/commit/422d2602916c8303b3c243cde71
820474e62d77d)
* Ideally there should be some non-js support but that can come later.
* Add ARIA roles to menus as recommended by Bootstrap 3 drop down patterns.
Rename 'more-nav' to 'masthead-menu'

* ‘more-nav’ is a class that should really only apply to the “More” tab
* Use a more descriptive and re-usable classname, ‘masthead-menu’, also
conforms to the ‘masthead-menu-item’ class and its aria role.
Don't hide certain tabs at small widths

* Previously we’d hide the “Corporate information” tab at tablet width
and below. This is bad form.
* The CSS has already been removed, but the class should be removed too.
Scope beta and impartiality styles
Shortcut ajax call when statistics are selected
Remove trailing whitespace
Publications#index retains params when redirecting to stats#index
Render tabs correctly in IE7, IE8

* IE7 is ~9% of current users, currently (on the live version) the tabs
aren’t displaying at all
* Use inline-block mixin for inline list to ensure it works in older
versions of IE