Add draft banner

[remibetin]

Related to #265

In the preview:

• Add a publication warning in header and footer: https://wai-website-theme.netlify.app/components/publication-warning/
• Allow editor notes to show up: https://wai-website-theme.netlify.app/components/editor-notes/

This will not affect live site.

[netlify[bot]]

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[remibetin]

QA done: ✅