Use GitLab snippets for code fences/blocks

Transform markdown code fences/blocks into GitLab snippets and embed

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Here is what Gitter code fences currently look like:

GitLab side

Snippet API: https://docs.gitlab.com/ee/api/snippets.html

I was thinking that we would have to add anonymous snippets for GitLab but then noticed GitHub removed their anonymous gist creation.

Maybe we can instead just open a URL with a GitLab snippet filled in ready to create for non-GitLab users -> gitlab-org/gitlab#199402