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.