Add merge request settings support for group level

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

What does this MR do and why?

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

specific changes

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. Database. Add three new columns for table namespace_settings (completed in this migration MR)
2. Backend. Add methods xxx_locked? and actual_xxx for the project model
3. Backend. Add methods xxx_locked? and actual_xxx for the namespace_settings model
4. Frontend. Add disabled logic to the Project > Settings > Merge requests > Merge checks
5. Frontend. Add Group > Settings > General > Merge requests > Merge checks page

How to set up and validate locally

Feature.enable(:support_group_level_merge_checks_setting)

Link: http://127.0.0.1:3000/groups//-/edit

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cc @prajnamas @icbd @zhzhang93 @JeremyWuuuuu @orozot @qt-gith