update structure of json

[Muskos]

We need to add id or code to the category. For example:

{
            "name": "OOP",
            "code": "oop",
            "sources": [{
                    "type": "videos",
                    "links": ["https://www.youtube.com/watch?v=rZcTaRU7AAw", "https://www.youtube.com/watch?v=zqxVI_kEdq8"]
                },
                {
                    "type": "playlists",
                    "links": ["https://www.youtube.com/playlist?list=PL6LDsbZOeyrx462VmH18qS0a9Dw9LwpSu"]
                },
                {
                    "type": "wiki",
                    "links": ["https://ru.wikipedia.org/wiki/%D0%9E%D0%B1%D1%8A%D0%B5%D0%BA%D1%82%D0%BD%D0%BE-%D0%BE%D1%80%D0%B8%D0%B5%D0%BD%D1%82%D0%B8%D1%80%D0%BE%D0%B2%D0%B0%D0%BD%D0%BD%D0%BE%D0%B5_%D0%BF%D1%80%D0%BE%D0%B3%D1%80%D0%B0%D0%BC%D0%BC%D0%B8%D1%80%D0%BE%D0%B2%D0%B0%D0%BD%D0%B8%D0%B5"]
                }
            ]
        }

[Muskos]

Also, we could add icons. We could add icon source, or use id (code) as icon's source