Remove attr_accessible from the user model

- This uses Whitehall's implementation of overriding attr_accessible,
  because Rails 4 errors when it encounters attr_accessible, but the
  gds-sso gem is still using attr_accessible. Found through @jennyd and
  https://github.com/alphagov/whitehall/blob/11b5293230c318ebca3444b5bc700841ca4810f6/lib/attr_accessible_noop.rb.