Support CloudFront geo header

In addition to supporting the CloudFlare header, we should add
support for the CloudFront one so that we can more easily switch
CDN providers should we need to.

bedrock/base/geo.py

@@ -21,7 +21,8 @@ def get_country_from_param(request):
 
 def get_country_from_header(request):
     """Return an uppercase 2 letter country code retrieved from the request header."""
-    country_code = valid_country_code(request.META.get("HTTP_CF_IPCOUNTRY"))
+    country_code = request.META.get("HTTP_CLOUDFRONT_VIEWER_COUNTRY", request.META.get("HTTP_CF_IPCOUNTRY"))
+    country_code = valid_country_code(country_code)
     if not country_code and settings.DEV:
         country_code = settings.DEV_GEO_COUNTRY_CODE