`Nokogiri::HTML.fragment` is a huge hack (a comment in the source code
admits this). The current behavior of `Email::Styles` is to try to
emulate `fragment` using nokogumbo, but it misses some edge cases. In
particular, meta tags in a email template don't make it through to the
final email.
Instead of treating the provided HTML as an indeterminate fragment, this
commit makes `Email::Styles` treat the HTML as a complete document. This
means that the generated HTML for an email will now always contain top
level structure (a doctype, html, head and body tags).
This new behavior is behind a hidden site setting for now and defaults
off.
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