allow cardinality of no limit

[Kyle Nitzsche]

There's a default cardinality of 6 when not using user settings for cardinality.

 There should be no default limit.

[Kyle Nitzsche (knitzsche)]

release 1.5.21 fixes this as follows:

If you declare a scope, category or keyword with a cardinality of 0, then 0 is passed, which means there is no limit on the number of results, for examlple:

        {
            "scope":
            {
                "id": "com.canonical.scopes.contacts_contacts",
                "local_id": "contacts",
                "category_title_use_incoming": "true",
                "cardinality": 0,
                "renderer":
                {
                    "components":
                    {
                        "art": "art",
                        "title": "title",
                        "subtitle": "published",
                        "attributes": "attributes"
                    },
                    "schema-version": 1,
                    "template":
                    {
                        "card-size": "medium",
                        "collapsed-rows": 1,
                        "category-layout": "horizontal-list"
                    }
                }

            }
        },