Note we're using a new sanitizer. Ideally we'd reuse the
`AdminWYSIWYGSanitizer`, but then code that would be correctly shown by
markdown-it (like the <h1> tag) wouldn't be shown on the web, which is
confusing. Ideally we would configure markdown-it to only allow the tags
present in the `AdminWYSIWYGSanitizer` and provide some kind of help
showing which tags are allowed.
