closes https://github.com/TryGhost/Product/issues/3818
- in Admin, when adding a recommendation, the URL is compared against all existing ones. If the URL is already recommended, the publisher is shown an error: "A recommendation with this URL already exists.". Protocol, www, query parameters and hash fragments are ignored during the URL comparison.
- on the backend, there is another uniqueness validation for the recommendation URL. This check is redundant when adding a recommendation from Admin, but helps to keep data integrity when recommendations are added through other paths (e.g. via the API)
