Add links to alternative json resources in Tariff pages

Enables API endpoint discovery
Serve API requests when requested with HTTP Accept header application/json
Log GOVUK-Request-Id header
Merge pull request #105 from alphagov/61656212-link-to-json-resource

61656212 link to json resource
Merge pull request #108 from alphagov/log_govuk_request_id

Log GOVUK-Request-Id header