Replace GFM Parser (redcarpet) with cmark

Description

Other repo services have begun to use cmark to parse markdown files (README, LICENSE, Documentation, Wiki, etc.). For example, GitHub now uses cmark to render markdown as per https://githubengineering.com/a-formal-spec-for-github-markdown/. Atlassian also uses cmark to render markdown as per https://github.com/atlassian/commonmark-java.

Non-GitHub features will have to be reimplemented, like Video, Math, and others.

Proposal

• Replace redcarpet with cmark
• Implement some features using GitHub's fork
• Bring over other features from GitLab/redcarpet to cmark

Links / references

• https://githubengineering.com/a-formal-spec-for-github-markdown/ (Article)
• https://github.github.com/gfm/ (Spec)
• https://github.com/github/cmark (GitHub fork of cmark with extensions)
• https://github.com/gjtorikian/commonmarker (Ruby implementation of cmark)