allow real JSON strings to be passed as custom

Try this (on local)

[Custom Level](http://localhost:3000/?game=tutorial&level=3&custom={%22name%22:%22My%20Custom%20Level%22,%22sprites%22:[{},{%22fill%22:%22purple%22},{%22kind%22:%22log%22,%22position%22:[1,0]}],%22goal%22:{%22clicks%22:4}})