Issue title: Netlify cms
 Issue body: ---
name: Bug report
about: Code blocks
---

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# Bug report

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**What is the current behavior?**
I dont know if this is for here or netlify cms specific but when i add a code block to site is saves as inline code ie `<html><body><p>example p tag</p></body></html>` on both netlify cms as well as in the starter site even tho in the cms i add line breaks

**If the current behavior is a bug, please provide the steps to reproduce.**


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**What is the expected behavior?**
`
<html>
<body>
<p>example p tag</p>
</body>
</html>
`

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**Other relevant information:**

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Node.js version:  v10.15.3
NPM/Yarn version: v1.13.0
Operating System: linux mint 19.1
Additional tools:

 Comments: 
 Comment 0: I don't think embedding code example is working in the current version of NetlifyCMS. @erquhart Can you confirm my suspicion? If not, is there a documentation on how to fix this? i can't seem to find anything related to this
 Comment 1: You need to add a gatsby plugin to handle code examples,[see how NetlifyCMS website is handling this](https://github.com/netlify/netlify-cms/blob/63582dcbc7920cfcc09551926d898060a689cb24/website/gatsby-config.js#L59), apart from that i found that they include the [prismjs style explicitly in the temaplate](https://github.com/netlify/netlify-cms/blob/63582dcbc7920cfcc09551926d898060a689cb24/website/src/templates/doc-page.js#L4)

 Comment 2: ok thanks!
{'source': 'GithubIssues', 'repo_name_with_owner': 'netlify-templates/gatsby-starter-netlify-cms', 'issue_id': 419138691, 'repo_stars': 1701.0}