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