Add feature flag for GraphQL rendering

This allows us to toggle the use of GraphQL by environment, in the same
way as Collections: alphagov/collections@bc1fedf.

app/lib/features.rb

@@ -0,0 +1,5 @@
+class Features
+  def self.graphql_feature_enabled?
+    ENV["GRAPHQL_FEATURE_FLAG"]
+  end
+end