Fix data:image CSP error and move content security policy rules to this repo

Author: AshGDS

config/initializers/content_security_policy.rb

@@ -1 +1,5 @@
-GovukContentSecurityPolicy.configure
+GovukContentSecurityPolicy.configure do |policy|
+  # Ensures the ability to render an OS map
+  img_policy_with_os_api = (policy.img_src + ["https://api.os.uk"] + [:data]).uniq
+  policy.img_src(*img_policy_with_os_api)
+end