Extend Markdown support

• Upgrade Markwon version to 2.0.0
• Upgrade Markwon version to 4.0.1
• Implement displaying labels in markdown.
• Implement displaying milestones in markdown.
• Implement displaying user mentions in markdown.
• Implement displaying link for issue in markdown.
• Implement displaying link for merge request in markdown.
• Implement displaying emojis in markdown.
• Implement displaying images in table cell.
• Implement clicks in markdown.
• Use default scaling images implementation instead of workaround from 2.0.0-SNAPSHOT.
• Parse non-marked links in text and wrap them with URL markdown tag

See the input character in https://gitlab.com/help/user/markdown#special-gitlab-references.