Also makes it work when given a parent element containing a
`.js-syntax-highlight` element so for dynamically-added things like
notes or Markdown previews, we can more accurately target just the
element we care about.
Instead of rendering this value server-side, we use Javascript and Gon
to apply the user's color scheme (or the default) to any syntax
highlighted code blocks.
This will make it easier to cache these blocks in the future because
they're no longer state-dependent.