Disable header check on preview

This header was introduced to ensure that no traffic was being
directed straight to the .cloudapps.digital domain. This was especially
useful for the non-production environments where access to the proper
domains is allowed for specific IP addresses while the cloudapps.digital
ones are open.

Moving to paas custom domains [1] will allow us to stop using the paas
proxies and, as a result, unbind the cloudapps.digital domain from our
apps.

This means that the X-Custom-Forwarder will become obsolete since all
our requests will be coming directly to our domain (albeit through
cloudfront) so any IP restriction can be implemented with a route
service [2].

1: https://docs.cloud.service.gov.uk/deploying_services.html#set-up-a-custom-domain-using-the-cdn-route-service
2: https://docs.cloud.service.gov.uk/deploying_services.html#route-services

app/config.py

@@ -406,7 +406,7 @@ class Preview(Config):
     LETTERS_SCAN_BUCKET_NAME = 'preview-letters-scan'
     FROM_NUMBER = 'preview'
     API_RATE_LIMIT_ENABLED = True
-    CHECK_PROXY_HEADER = True
+    CHECK_PROXY_HEADER = False
 
 
 class Staging(Config):