Settings looked at `domains.yml` when users were making go live requests
or email branding requests.
This will allow us to remove the `domains.yml` file, by using
information about organisations that is now stored in the database
instead.
This is the first step of replacing the `domains.yml` file.
In order to replicate the same functionality we get from the
`domains.yml` file and its associated code this commit adds a
`Organisation` model. This model copies a lot of methods from the
`AgreementInfo` class which wrapped the `domains.yml` file.
It factors out some stuff that would otherwise be duplicated between the
`Organisation` and `Service` model, in such a way that could be reused
for making other models in the future.
This commit doesn’t change other parts of the code to make use of this
new model yet – that will come in subsequent commits.
