Allow for global default of "Only allow merge requests to be merged if all discussions are resolved"
### Description
Right now you need to enable this feature per project. When you have a large majority of your teams wanting to consume this feature, this becomes cumbersome to remember.
### Proposal
Allow for a global, default, setting for this. (Essentially making it opt-out vs opt-in)
### Links / references
### Documentation blurb
#### Overview
What is it?
Why should someone use this feature?
What is the underlying (business) problem?
How do you use this feature?
#### Use cases
Who is this for? Provide one or more use cases.
### 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