Add setting to only allow merge requests to be merged when all discussions are resolved

_Originally opened at !6385 by @rodolfoasantos._

- - -

## What does this MR do?
Based on #20968 this merge request adds setting only to allow merge requests to be merged when all discussions are resolved. 

## Are there points in the code the reviewer needs to double check?
Check if there are other points to add the resolved discussion setting

## Why was this MR needed?
Add the possibility to configure the project to only accept merge request when all discussions are resolved

## Screenshots

![only_allow_merge_if_all_discussions_are_resolved](/uploads/9388db9421da0214590ffab6fb29f985/only_allow_merge_if_all_discussions_are_resolved.png)

![only_allow_merge_if_all_discussions_are_resolved_msg](/uploads/b1bba0c72ad67d3a1b34718baa08526e/only_allow_merge_if_all_discussions_are_resolved_msg.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Closes #20968

See merge request !7125