Expose organisations in the user.json

This will enables apps to know what organisations a user is a member of.

The slugs will be sourced from the Whitehall Organisations API: http://whitehall-admin.dev.gov.uk/api/organisations

I have successfully tested for compatibility with Publisher and Whitehall. I don't anticipate any problems, but I will test integration with apps that don't use gds-sso (eg Performance Platform) on preview.