Use the project-level/MR-level approval rules public API in web UI

Summary

This started from the discussion in: !13895 (comment 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 !23107 (comment 272139093)).

Risks

Creating project approval rules via web UI might fail so we need to ensure that it works.

Involved components

• gitlab-rails