Filter to display Merge Requests with unresolved discussions
### Description
Filter to display Merge Requests with unresolved discussions
Add a new filter to limit the results in the Merge Requests list to only those that have unresolved discussions
### Documentation blurb
#### Overview
What is it?
A new filter for Merge Requests
Why should someone use this feature?
To discover Merge Requests that still require actions
How do you use this feature?
Choose the filter when browsing the Merge Request list
#### Use cases
Who is this for? Provide one or more use cases.
Project maintainers to find in progress, incomplete Merge Requests
### 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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