Use different colours for each environment

https://www.pivotaltracker.com/story/show/112786779

> There's an emerging convention on admin apps, to have a red strip atop the
> page, also to have a different colour for preview environment... so let's
> adopt that and see how it feels. Red for prod and gold for preview.

This commit adds config so that:
- yellow locally
- orange on preview and staging
- red on live

It will not actually work until each AWS environment uses the right config, but
can be tested locally by setting the environment variable manually, eg:
`export HEADER_COLOUR='#F47738'`

app/templates/admin_template.html

@@ -5,6 +5,9 @@
   <!--[if gt IE 8]><!-->
   <link rel="stylesheet" media="screen" href="{{ asset_path }}stylesheets/main.css" />
   <!--<![endif]-->
+  <style>
+    #global-header-bar { background-color: {{header_colour}} }
+  </style>
   <!--[if IE 6]>
   <link rel="stylesheet" media="screen" href="{{ asset_path }}/stylesheets/main-ie6.css" />
   <![endif]-->