[gatsby-transformer-remark] Recursively add _PARENT to all sub-objects in frontmatter

[Seavenly]

This is a fix in reference to #2286 where the _PARENT property is not created on nested sub-objects which causes GraphQL to throw an error when looking for references to nested File nodes.

[KyleAMathews]

Deploy preview failed.

Built with commit b1bf0ae

https://app.netlify.com/sites/using-glamor/deploys/59e3b82fdf99536a644d18c1

[gatsbybot]

Deploy preview ready!

Built with commit b1bf0ae

https://deploy-preview-2466--gatsbygram.netlify.com

[KyleAMathews]

Ah perfect! I realized this was needed after people stopped complaining about one level down in frontmatter not working but that two plus wasn't working :-)

Thanks for the fix!

[jlengstorf]

Hiya @Seavenly! 👋

This is definitely late, but on behalf of the entire Gatsby community, I wanted to say thank you for being here.

Gatsby is built by awesome people like you. Let us say “thanks” in two ways:

1. We’d like to send you some Gatsby swag. As a token of our appreciation, you can go to the Gatsby Swag Store and log in with your GitHub account to get a coupon code good for one free piece of swag. (We’ve got t-shirts and hats, plus some socks that are really razzing our berries right now.)
2. If you’re not already part of it, we just invited you to join the Gatsby organization on GitHub. This will add you to our team of maintainers. You’ll receive an email shortly asking you to confirm. By joining the team, you’ll be able to label issues, review pull requests, and merge approved pull requests.

If you have questions, please don’t hesitate to reach out to us: tweet at @gatsbyjs and we’ll come a-runnin’.

Thanks again! 💪💜