Improve handling of Duplicate issues

### Description In software with long history, new reported issues are a higher percentage of duplicates than actual new issues. For that reason, software like Bugzilla etc. makes duplicates a first class citizen. ### Proposal To have UI/UX for duplicates handling. For example in Bugzilla duplicates UI handling is mostly part of the UI of the generic relationships between issues. There it's possible to: * Create duplicates from the UI (look at the bottom of the images):   * There are hints about the duplicate relationship between the bugs:    * and lately, you can filter them out in search so we don't end up looking at possible unrelated duplicates. I guess the UI/UX for GitLab will be different, but I hope the pictures are clear enough for how it's used in Bugzilla. ### Feature checklist Make sure these are completed before closing the issue, with a link to the relevant commit. - [ ] [Feature assurance](https://about.gitlab.com/handbook/product/#feature-assurance) - [ ] Documentation - [ ] Added to [features.yml](https://gitlab.com/gitlab-com/www-gitlab-com/blob/master/data/features.yml)

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