Markdown highlighting from the edit page is not compliant with commonmark

Summary

Markdown syntax highlighting in the edit file view is not compliant with the commonmark spec, see: image

The line that goes #grilo on GIMPNet is highlighted as if it was a header, when this is no longer the case with commonmark.

Steps to reproduce

  1. Edit a markdown file (e.g. https://gitlab.com/gitlab-org/gitlab-ce/blob/master/README.md from the page)
  2. Paste #grilo on GIMPNet or type a header without a space onto a new line.

What is the current bug behavior?

Line is highlighted

What is the expected correct behavior?

Line should not be highlighted