Move FCO payment services to service domains

The following FCO services are moving from GOV.UK onto their own
service.gov.uk subdomains.

- pay-foreign-marriage-certificate
- pay-legalisation-drop-off
- pay-legalisation-post
- pay-register-birth-abroad
- pay-register-death-abroad
Merge pull request #9 from alphagov/fco-move-to-service-domain

Move FCO payment services to service domains