Add logging of variant for navigation test

This will enable us to determine which variant a log entry refers to.  This
will help with allowing us to measure performance and track errors.

Author: sihugh

config/initializers/logstasher.rb

@@ -0,0 +1,5 @@
+if Object.const_defined?('LogStasher') && LogStasher.enabled
+  LogStasher.add_custom_fields do |fields|
+    fields[:govuk_content_pages_variant] = request.headers['GOVUK-ABTest-ContentPagesNav']
+  end
+end