Create single source of truth for MR reviewer suggestions

Objective

Suggest reviewers in one location for gitlab-org/gitlab merge requests.

Challenge

Currently necessary reviews are split between the Roulette suggestions and MR Approvals widget based on what type of approval is needed. As an example merge requests that require a Manage::Auth review are sometimes caught late because the review isn't apart of Roulette.

Proposal

Suggested reviewers from required code owner approval sections in the Roulette comment via the approval_state api call and use the Roulette availability signals for identifying a DRI for review.