Add inheritance dropdown selection to group-level MR approval rules

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 - Apply to - at the group level.

Proposal

This should be behind a feature flag.

Add the inheritance dropdown selection Apply to to the group level.

• Apply settings to all projects (new and existing)
• Apply settings only to new projects
• Apply settings only to existing projects

Stretch Goal: Choosing one of these options should apply the necessary logic to the subgroups and projects within the parent/root group.

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.