Add Gravity Forms Support

[gregrickaby]

Need to be able to display and process Gravity Forms.

Kellen suggested we try: https://www.npmjs.com/package/gatsby-source-gravityforms

The WPGraphQL source plugin and a sample form is setup on Lab. Also, there is a form in https://gatsby.wdslab.com/2019/08/30/hello-world/ and the GraphQL fragment is being passed.