Switch tab and masthead colours

* Try and match other admin apps a little more, use a darker
background, and lighten the tabs.
Add crown icon and app name to header
* Match other admin applications
* Clarify name of application
Match other apps environment indicators
Indent crown for better alignment with tabs
Introduce coloured environment bar

* Match other admin apps with a 5px coloured bar stretching across the
header
* Because of tabs this is at the top of the masthead rather than the
bottom
Merge pull request #1651 from alphagov/dark-header

Blacken Whitehall admin header