Draft: Add merge request settings support for group (EE)

This MR completes this issue and this epic at the same time: #372040 (closed) & &8704 (closed)

In order to facilitate the code review, I divided it into two MRs according to CE EE:

• CE: Add merge request settings support for group (CE)
• EE: Add merge request settings support for group (EE)

Final goal of these two MRs

Add Merge requests > Merge checks settings support for group level.

What does this CE-MR do and why?

The changes revolve around these three configuration items:

only_allow_merge_if_pipeline_succeeds
allow_merge_on_skipped_pipeline
only_allow_merge_if_all_discussions_are_resolved

1. Backend: Add two methods actual_xxx and xxx_locked? to namespace_settings model in EE
2. Backend: Add two methods actual_xxx and xxx_locked? to projects model in EE
3. Frontend: Add a Merge requests page on the group settings page, where MR checks are included (The main content of MR)

How to set up and validate locally

Feature.enable(:support_group_level_merge_checks_setting)