Labels support color names in backend

What does this MR do?

Support color names (according to Mozilla CSS color keywords) for labels in backend.

Are there points in the code the reviewer needs to double check?

https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/bitbucket_import/importer.rb#L134

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #28424 (closed)