github_flavored_markdown: Add GFM-like sanitization via bluemonday.

This way, the output of Markdown() more closely matches the output of
the GitHub Markdown API endpoint.
Add some basic sanitization tests.
Remove WriteMarkdownGfmAsHtmlPage because it's no longer helpful;
should directly use one of WriteGitHubFlavoredMarkdownViaLocal or
WriteGitHubFlavoredMarkdownViaGitHub.