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
issue