Assign user as a reviewer when they submit a review

Proposal

When a user (who is not assigned as a reviewer) submits a review we will automatically added them as a reviewer.

Observing

We want to understand the relationship between approvals and reviewers better to see if it is a 1:1 majority of the time.

We will be measuring "After removal of the approval widget, the number of unassign reviewer will not increase from the baseline" https://gitlab.com/gitlab-org/gitlab/-/issues/580389

Today we have the "Approved by" area that shows all approvals given on the merge requests but this list of approvals could come from users who are not listed as reviewers.

Given the data, we could make decisions to simplify the information layout deduplicating "Approved by" and the Reviewers list of those who have approved.

Changes

1. Add a feature flag for this featureenhancement
2. Change done on the backend, instead of just a find_by method change to find_or_create_by to create the reviewer object if the user is not a reviewer
3. Only show the text if the user is not assigned as a reviewer

Design

Figma