Allow managing editors access to featured services and guidance.

They are limited to their own organisation.

https://www.pivotaltracker.com/story/show/76712400
I don't think that's necessary. Testing the two cases (can and can't) as you have here with real users instead of stubbing stuff out would be good enough. e.g.

```ruby
  view_test "Prevents unauthorized management of featured services and guidance" do
    organisation = create(:organisation)

    login_as :policy_writer
    get :edit, id: organisation
    refute_select "legend", text: "Featured services and guidance"

    login_as :gds_editor
    get :edit, id: organisation
    assert_select "legend", text: "Featured services and guidance"
end
```