[BE] Related Issues

Provides a manual way of relating one issue to another and present those connections on a Widget on Issues page.

Obs.: This is a reopened MR of the already reviewed https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1719

Note that !1719 (merged) was reverted on https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2082 once the whole feature couldn't make it to %9.3.

Docs on https://gitlab.com/gitlab-org/gitlab-ee/blob/4a641a029102d099b57362988a2c3af35893365d/doc/user/project/issues/related_issues.md

FE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1797

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• Changelog entry added, if necessary
• [ ] 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?

Related #2001 (closed)