At the moment we have to update a YAML file and deploy the change to get
a new domain whitelisted.
We already have a thing for adding new domains – the organisation stuff.
This commit extends the validation to look in the `domains` table on the
API if it can’t find anything in the YAML whitelist.
This has the advantage of:
- not having to deploy code to whitelist a new domain
- forcing us to create new organisations as they come along, so that
users’ services automatically get allocated to the organisation once
their domain is whitelisted
8835486d4e