draft 07 JSON schema support

[ghost]

I noticed that the form for nested items is not generated properly (if properties attribute is not used for the nested elements), e.g. nested fields are not rendered for the following schema which is taken from https://json-schema.org/learn/miscellaneous-examples.html.

{
  "$id": "https://example.com/arrays.schema.json",
  "$schema": "http://json-schema.org/draft-07/schema#",
  "description": "A representation of a person, company, organization, or place",
  "type": "object",
  "properties": {
    "fruits": {
      "type": "array",
      "items": {
        "type": "string"
      }
    },
    "vegetables": {
      "type": "array",
      "items": { "$ref": "#/definitions/veggie" }
    }
  },
  "definitions": {
    "veggie": {
      "type": "object",
      "required": [ "veggieName", "veggieLike" ],
      "properties": {
        "veggieName": {
          "type": "string",
          "description": "The name of the vegetable."
        },
        "veggieLike": {
          "type": "boolean",
          "description": "Do I like this vegetable?"
        }
      }
    }
  }
}

See screenshot: