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'`

config.py

@@ -39,6 +39,8 @@ class Config(object):
     MAX_CONTENT_LENGTH = 10 * 1024 * 1024  # 10mb
     UPLOAD_FOLDER = '/tmp'
 
+    HEADER_COLOUR = '#FFBF47'  # $yellow
+
 
 class Development(Config):
     DEBUG = True
@@ -53,10 +55,19 @@ class Test(Development):
     WTF_CSRF_ENABLED = False
 
 
-class Live(Config):
+class Preview(Config):
     DEBUG = False
     HTTP_PROTOCOL = 'https'
     SESSION_COOKIE_SECURE = True
+    HEADER_COLOUR = '#F47738'  # $orange
+
+
+class Staging(Preview):
+    pass
+
+
+class Live(Staging):
+    HEADER_COLOUR = '#B10E1E'  # $red
 
 
 configs = {