Extract code owners from a CODEOWNERS file

What does this MR do?

This MR adds the possibility to set codeowners for files using a CODEOWNERS file that's can be located in .gitlab, docs or the root of the repository.

The syntax of the CODEOWNERS file is similar to that of .gitignore, so patterns can be used to match paths. Users can be linked to patters using their @username or email address.

When a file matches multiple entries in the CODEOWNERS-file, the users from all entries are displayed.

The code owners are displayed on a blob page like this:

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/6961

CE-port in: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21309

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
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?