closes https://github.com/TryGhost/Product/issues/3955
- also, moved recommendations code to the "membership" folder for
consistency with the UI
- also, renamed "yarn test:slowmo" to "yarn test:acceptance:slowmo" for
consistency in the naming
- also, removed "limit: 1" when checking whether a recommendation URL
already exists. The limit defaults to 5 on the API side, and allowing
more than 1 result will lead to a more thorough check
