WIP: Validate uniquness of labels for issues and MRs on database level

What does this MR do?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/28519

TODO

• Create a migration to remove the old index
• Wrote the SQL to remove duplicates
• Create a migration to add a new index['target_id', 'target_type', 'label_id']
• Make it follow our new Migration Style Guide.

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?

https://gitlab.com/gitlab-org/gitlab-ce/issues/28519