Remove use of commonmarker gem for import

https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/github_import/markdown_text.rb#L23-L34 uses CommonMarker.render_doc in order to parse markdown for extracting attachments during import from GitHub.

We no longer use the the commonmarker gem for parsing markdown, but instead use gitlab-glfm-markdown - see Banzai::Filter::MarkdownFilter

The was removed from the Gemfile in Remove CMARK_ENGINE used in older markdown rend... (!192279 - merged) (although kept in by graphql-docs depending on it), but it's still used by the import code, so was readded in Add back commonmarker (!193481 - merged).

In any case, we need to rewrite fetch_attachments to use our gem rather than commonmarker.