[Discovery] Add Prevent approval of merge requests by merge request author 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.
User experience goal
group owner can configure this setting -
Prevent approval of merge requests by merge request author - at the group level.
This should be behind a feature flag.
Add the setting
Prevent approval of merge requests by merge request author to the group level.
Enabling this setting should inherit it down to all projects, but still allow
maintainers to change this setting.
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.