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)
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.
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
Collapse tabs when the screen isn't wide enough

* Collapse tabs into a stacked view using media queries
* Breakpoint determined by the point at which wrapped text within tabs
causes problems (slightly above the usual tablet breakpoint)
Correct alignment of environment indicator

* Shift margins in IE7 for correct alignment
* Remove old media query, the indicator margin doesn’t need to change
at smaller widths
Merge pull request #1611 from alphagov/tab-clarity

Simplify markup and styling of admin header