Add Prevent approval of merge requests by merge request committers to group level

Problem to solve

In pursuit of the holistic solution outlined in &4367, this issue is a smaller implementation issue to iteratively move elements of project-level merge request approval rules to the group-level.

Please see &4367 for the discovery and implementation discussion.

Intended users

• Cameron (Compliance Manager)
• Sidney (Systems Administrator)

User experience goal

A group owner can configure this setting - Prevent approval of merge requests by merge request committers - at the group level.

Proposal

This should be behind a feature flag.

Add the setting Prevent approval of merge requests by merge request committers to the group level.

Stretch Goal: Enabling this setting should inherit it down to all projects, but still allow maintainers to change this setting.

Implementation plan

frontend - 1️⃣

• Add Prevent approval of merge requests by merge request committers checkbox, which takes account of higher-level restrictions (instance or group when used on a project)
• Add the ability to save this value via the store
• Check that success and errors are being handled by the stores existing process
• Add tests

Further details

We should build this behind a feature flag so we can complete the entire implementation before enabling that flag by default. This should help us ensure there's a solid UX that accounts for the different preferences our users have around inheritance and enforcement.