text: Title: Correct GraphQL field for wagtail RichTextField
  Body: ```python
  from wagtail.core.fields import RichTextField
  
  @register_snippet
  class TestSnippet(models.Model):
      title = models.CharField(max_length=50)
      description = RichTextField(
          blank=True
      )
  
      panels = [
          FieldPanel('title'),
          FieldPanel('description'),
      ]
  
      graphql_fields = [
          GraphQLString('title', required=True),
          GraphQLString('description', required=True),
      ]
  ```
  
  I have been using `GraphQLString` for wagtail `RichTextField` which doesn't seem to be the most accurate way..as Richtext field output should ideally be passed through richtext method for the rendered content. E.g. In template there is richtext filter and [expand_db_html](https://github.com/wagtail/wagtail/blob/571b9e1918861bcbe64a9005a4dc65f3a1fe7a15/wagtail/core/rich_text/__init__.py#L24) method.
  
  One example is when using apostrophe `'`  inside richtext fields, it returns code instead of the string. 
  
  ```shell
  In [62]: object.text
  Out[62]: '<p data-block-key="t4jac">dc csdc &#x27;This is the string&#x27;</p>'
  
  ```