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
Use module accessors to store the API clients

This reduces duplication a little bit
Enable/disable support-api with a feature flag

This is a very temporary step so that `feedback` app deploys aren't
blocked by needing to have the `support-api` running smoothly in production.
Merge pull request #132 from alphagov/switch-service-feedback-to-support-api

Switch service feedback to 'support-api'