CT-1391 map third_party true/false to yes/no
CT-1391 map third_party true/false to yes/no

This is necessary for the boolean value that third_party is to present itself on
the new page form as "Yes" or "No", and for the chosen option to persist if
there are validations errors and the new page has to be redisplayed.

ministryofjustice/correspondence_tool_staff,ministryofjustice/correspondence_tool_staff,ministryofjustice/correspondence_tool_staff,ministryofjustice/correspondence_tool_staff,ministryofjustice/correspondence_tool_staff