Display national statistics logo on stat announcement show page

National statistics are a very specific, accredited kind of statistic.
We should display the logo.
Merge pull request #1556 from alphagov/national-stats-announcement-logos

Display national statistics logo on stat announcement show page
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
Add missing full stop
Merge pull request #1558 from alphagov/add-fullstop-in-request-alternative-format-callout

Add missing full stop in "alternative format" callout
Merge pull request #1557 from alphagov/environment-indicator

Distinguish between Whitehall environments
Revert closed roles merge: #1523
Merge pull request #1559 from alphagov/revert-roles

Revert closed roles merge: #1523
Typo ex chancellors page
Merge pull request #1561 from alphagov/typo-past-chancellors

Typo ex chancellors page
Bump Plek gem

The new version gives us Plek.new.website_uri
Use shorthand for conditional assignment

We can rely on these ENV vars being nil rather than blank as they are
highly unlikely to be set with an empty value
Whitehall.asset_host goes via Plek

We shouldn't need to access ENV vars from within the app. Plek should be
giving us exactly what we need.
Change default static asset serving in dev mode

Up to now, whitehall has defaulted to serving assets from preview in
development mode. This is the opposite of how all the other apps in our
stack works. This reverses that so that whitehall expects assets to be
served from wherever Plek says they will be, but also allows the static
server to be overridden directly with an ENV var (STATIC_DEV) if
required. For convenience for those developers who are running whitehall
outside of the VM, startup.sh has been updated to set this ENV var so
assets are served from preview.
Remove unused initializer

This initializer is no longer used as staging gets the right asset host
as of alphagov-deployment commit 12c3eefb6777881fea48c24638b04db2f6f3c07d
There should only be one Whitehall#admin_host

Plek gives us the correct host based on the current environement.
Whitehall.public_host derived from Plek

Up to now, we've set Whitehall.public_host in an initializer that is
overridden per-environment during deploy. Plek is the source of all
truth so we do not need to do this.
Remove more hardcoded URLs

In most cases, the default Whitehall.url_maker will give us what we need
to generate both public and admin urls. Some tests required slight
adjustment to reflect the slight difference in the newly implicit
protocol (derived from the environment) and the previously explicit one
hardcoded into the source.