refs https://github.com/TryGhost/Team/issues/1047
Rendering segmented emails uses `cheerio` to parse and re-render the html but this had a side-effect of converting the `$#39;` char code to the more modern `$apos;` code resulting in Outlook not understanding quotes inside inlined CSS and showing the raw char code if it appeared in the email contents.
- extracted our handling of the unsupported char codes from the main email html generation into a function so that it can be re-used when generating segmented html
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