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/__init__.py

@@ -100,7 +100,10 @@ def init_app(app, config_overrides):
 
     @app.context_processor
     def inject_global_template_variables():
-        return {'asset_path': '/static/'}
+        return {
+            'asset_path': '/static/',
+            'header_colour': app.config['HEADER_COLOUR']
+        }
 
 
 def convert_to_boolean(value):