Enable CommonMark for files and wikis

What does this MR do?

This enables repository (.md) files and wiki files to be rendered by CommonMark by default, instead of RedCarpet. We've added the ability to view a file using the legacy RedCarpet rendering by adding the legacy_render=1 parameter. For example:

https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/README.md?legacy_render=1

This is for transitioning only, and will be removed in a future release when RedCarpet is completely removed.

What are the relevant issue numbers?

#43011 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21509
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides