Add environment message to admin header

* Show a red, orange or grey message depending on environment
* Message set at deploy time by an initialiser, following the same
patterns as publisher
Add environment based favicons
Keep env message margins consistent at thin widths

* At thin widths the main nav collapses down and the margins are
reduced, use a media query to keep them consistent
* Use the Bootstrap 2 breakpoint specified in
http://getbootstrap.com/2.3.2/scaffolding.html#responsive
Merge pull request #1557 from alphagov/environment-indicator

Distinguish between Whitehall environments