Allowing LDAP groups to be configure as Approvers for merge requests.
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### Description
When using Gitlab in an enterprise environment with may developers there is a need to restrict who can approve merge requests as well as minimizing administration of Gitlab it self. A solution to this would be to allow LDAP groups to be used as merge request approvers. This will have the benefit of a one time configuration of the gitlab project(s) and any further administration is done in LDAP where users can be added/removed from the merge requests approvers role.
#### Use cases
Users with many projects and many developers that need ease of use and low administration overhead.
### 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)
issue