feat: use mostly markdown for pr bodies (#1118)

Existing solution uses HTML bodies for PR descriptions, as that was the easiest way to get consistency between GitHub and GitLab. However, VSTS supports only markdown so we needed to refactor how this is done. Now, GitHub PR bodies uses only minimal HTML (for summary/details) while GitLab PR bodies are converted to HTML using GitHub flavoured markdown for maximum compatibility. VSTS will be able to strip out the minimal markdown.

Closes #1018