Integrate csslab

What does this MR do?

• Add csslab as a dependency
• Integrate markdown type into gitlab-ce
  • Update style in markdown file (when viewed in repo view)
  • Wiki
  • Snippet Markdown

How to test MR (aka enable csslab)?

1. yarn install
2. Go to rails console (rails c)
3. Enter the command Feature.enable(:csslab)
4. Refresh your page, csslab should now be loaded on the page!

Note: Here is a snippet with all the markdown stuff: https://gitlab.com/gitlab-org/csslab/snippets/1705683

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/csslab/issues/1

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• 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
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer