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
• Documentation
• Added to features.yml