Orphan list bug

PT story: https://www.pivotaltracker.com/story/show/79488584

- Fixed all three bugs
- Introduced features to test the fix
- Introduced specs to ensure an orphan list can only be created for an active partner
- Introduced orphan_list factory
I couldn't find a clean way to prevent rendering the view. @NikitaAvvakumov Do you have any ideas?

In addition to how I did it here, I found another possibility as in here: https://gist.github.com/motasem-salem/ac76bec4d0404830bd0d

By passing `blank_slate_link: nil`, I can exploit the unhandled branch in https://github.com/activeadmin/activeadmin/blob/9cfc45330e5ad31977b3ac7b2ccc1f8d6146c73f/lib/active_admin/views/pages/index.rb#L147

I ended up deciding on doing it this way because it seemed less "hacky" and at least I'm setting something (`params[:q]`) they are expecting and checking.