When we’re doing user research we often:
- start the task by inviting the participant to a service on Notify
- have them use a prototype version of the admin app, hosted on a
different domain
Currently we can’t do both of these things together, because the invite
emails always send people to `notifications.service.gov.uk` (because
it’s the API that sends the emails, and the prototype admin app points
at the production API).
This commit changes the API to optionally allow an instance of the admin
app to specify which domain should be used when generating invite links.