Add toggle to resolve all threads in a MR review workflow
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<!-- The first four sections: "Problem to solve", "Intended users", "User experience goal", and "Proposal", are strongly recommended, while the rest of the sections can be filled out during the problem validation or breakdown phase. However, keep in mind that providing complete and relevant information early helps our product team validate the problem and start working on a solution. -->
### Problem to solve
<!-- What problem do we solve? Try to define the who/what/why of the opportunity as a user story. For example, "As a (who), I want (what), so I can (why/value)." -->
An idea from [Twitter](https://twitter.com/marcosnils/status/1275062152693350401), new issue as suggested in https://gitlab.com/groups/gitlab-org/-/epics/3607#note_366584129.
A button/toggle to resolve all threads on an MR. The story behind this could be a separate commit addressing all the discussed changes, depending on the workflow. Or, if there are numerous remarks but they all are not necessary, this comes in handy as well.
This could lean into ["In a MR, "Please resolve these threads" should be a hyperlink pointing to the threads to be resolved"](https://gitlab.com/gitlab-org/gitlab/-/issues/215174).
### Intended users
* [Sasha (Software Developer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sasha-software-developer)
* [Devon (DevOps Engineer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#devon-devops-engineer)
* [Rachel (Release Manager)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#rachel-release-manager)
### User experience goal
<!-- What is the single user experience workflow this problem addresses?
For example, "The user should be able to use the UI/API/.gitlab-ci.yml with GitLab to <perform a specific task>"
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Imagine a merge request review workflow where N comments have been left, and all changes are applied. Instead if clicking each review thread one by one and resolve it, a global toggle with "resolve all threads" helps speed this up.
### Proposal
<!-- How are we going to solve the problem? Try to include the user journey! https://about.gitlab.com/handbook/journeys/#user-journey -->
Add a new toggle into the UI which fires the action in the background. It may need an additional API action to avoid multiple requests and do this in a single transaction.
### Further details
<!-- Include use cases, benefits, goals, or any other details that will help us understand the problem better. -->
Should this only apply to new comments or everything? A discussion point is to avoid "abuse" of this feature, where discussions are just turned down in order to merge the MR more quickly.
Before adding a PoC, I'd suggest a discussion/feedback on the matter.
### Permissions and Security
<!-- What permissions are required to perform the described actions? Are they consistent with the existing permissions as documented for users, groups, and projects as appropriate? Is the proposed behavior consistent between the UI, API, and other access methods (e.g. email replies)?-->
Applies to owner, maintainer and developer only.
### Documentation
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* If this feature requires changing permissions, update the permissions document. See https://docs.gitlab.com/ee/user/permissions.html -->
- Requires a docs section & screenshots for usage.
- Requires permission documentation.
### Availability & Testing
<!-- This section needs to be retained and filled in during the workflow planning breakdown phase of this feature proposal, if not earlier.
What risks does this change pose to our availability? How might it affect the quality of the product? What additional test coverage or changes to tests will be needed? Will it require cross-browser testing?
Please list the test areas (unit, integration and end-to-end) that needs to be added or updated to ensure that this feature will work as intended. Please use the list below as guidance.
* Unit test changes
* Integration test changes
* End-to-end test change
See the test engineering planning process and reach out to your counterpart Software Engineer in Test for assistance: https://about.gitlab.com/handbook/engineering/quality/test-engineering/#test-planning -->
Tests need to ensure that all threads are resolved, nothing is left, and MR can be merged then (if the requirement is enabled in project settings).
### What does success look like, and how can we measure that?
<!-- Define both the success metrics and acceptance criteria. Note that success metrics indicate the desired business outcomes, while acceptance criteria indicate when the solution is working correctly. If there is no way to measure success, link to an issue that will implement a way to measure this. -->
- Saved cycles in MR reviews.
- Users sharing this feature on social.
### What is the type of buyer?
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In which enterprise tier should this feature go? See https://about.gitlab.com/handbook/product/pricing/#four-tiers -->
This should be available for everyone, Core and Enterprise. Reason: This improves the MR review workflow and helps developers, similar to [batch review comments](https://docs.gitlab.com/ee/user/discussions/index.html#merge-request-reviews) now also moved to Core with 13.1. Adding this completes the workflow.
### Is this a cross-stage feature?
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### Links / references
cc @danielgruesso @mvanremmerden
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