This module is now the sole source of knowledge for *how* we render
Markdown (and GFM).

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.

It was redundant because `gfm` also took options.

Prior, CI seemed to be freezing after running these specs.

This file's about to get much bigger and this removes some of the extra
noise.

Not to be confused with the RSpec `type: :feature` tag, this tag is used
by the `spec:feature` Rake task for filtering/grouping specs.

Forces a `rel="nofollow"` attribute on all external links.

It was already working, just want to make sure we don't break it in the
future.