Improve viewing a CODEOWNERS file

Use an auxiliary blob viewer to display the CODEOWNERS file.

• Link the usernames mentioned in the codeowners file to their profile
• Link to the codeowner documentation

/cc @DouweM @jramsay

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The following discussion from gitlab-ee!6916 should be addressed:

• @DouweM started a discussion: (+1 comment)

  Now I'm thinking it would be cool if we also had an auxiliary viewer for the actual CODEOWNERS file, that would link to the docs on the feature, like the one we have for .gitlab-ci.yml. Something to add in a follow-up MR if you have time before the freeze? :)