Use the project-level/MR-level approval rules public API in web UI
## Summary This started from the discussion in: https://gitlab.com/gitlab-org/gitlab/merge_requests/13895#note_203359507. We added public API for project-level/MR-level approval rules. We are also keeping the private API so the web UI can continue to work. ## Improvements - Use the project-level/MR-level approval rules public API in the web UI. - Remove the private API as it'll be a duplicate of the public API. - Look into removing the need to filter project level approval rules (`Project#visible_approval_rules`) in API responses (see https://gitlab.com/gitlab-org/gitlab/merge_requests/23107#note_272139093). ## Risks Creating project approval rules via web UI might fail so we need to ensure that it works. ## Involved components - gitlab-rails
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