Push 'referrer' handling to base Ticket class

This allows referrer handling to be used for service feedback
Referrer is captured with service feedback

This allows service managers to put the service feedback in context
of where the user came from, in the case that a transaction has several exit
points.
Merge pull request #131 from alphagov/capture-referrer-for-service-feedback

Capture referrer for service feedback
Service feedback: switch from support to support-api
Bump gds-api-adapters

This picks up the `support-api` interface.