Add linting test around GDS::SSO User model
Add linting test around GDS::SSO User model

Ensures the User model has been implemented fully, and will
fail if the SSO gem is updated, and the implementation changed,
but not updated in the app.

See: https://github.com/alphagov/gds-sso/pull/58

alphagov/policy-publisher,alphagov/policy-publisher,alphagov/policy-publisher,alphagov/policy-publisher