Add related issues widget to MR

What does this MR do and why?

This MR adds a related work items widget to the MR sidebar. Currently, there is no way to link/unlink work item manually but it's planned for the next iteration.

References

• https://gitlab.com/groups/gitlab-org/plan-stage/-/work_items/456
• https://gitlab.com/groups/gitlab-org/plan-stage/-/work_items/454

Screenshots or screen recordings

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:mr_related_work_items)

2. Visit any MR page such as http://127.0.0.1:3000/flightjs/Flight/-/merge_requests/10
3. Add some related items to MR by editing its description with key words like Closes or Related to
4. Check the sidebar for Work Items widget.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.