Render labels correctly in content editor

What does this MR do and why?

Add support for rendering labels correctly in content editor, including their color, text color and scoped labels.

Note that scoped labels are a GitLab Premium feature.

Screenshots or screen recordings

How to set up and validate locally

1. Open any wiki page. Edit it.
2. In the content editor, type ~ to bring in labels suggestions.
3. The labels should be styled like they are everywhere else on GitLab.
4. Save the document. The label styles should persist even after save.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.