Ensure suggested reviewers order is preserved

What does this MR do and why?

Ensure the suggested reviewer's order is preserved.

This commit fixes the order issue on suggested reviewers caused by sorting in the frontend and order skipping in the backend.

• Remove alphabetical sort by user's name in front-end
• Add ordinality on suggested usernames in SQL in model

The front-end feature is gated behind a feature flag suggested_reviewers_control which is disabled by default.

Screenshots or screen recordings

Before	After

🐘 Database

Query

SELECT "users".*
FROM "users"
    INNER JOIN "project_authorizations" ON "users"."id" = "project_authorizations"."user_id"
    JOIN UNNEST(
        ARRAY ['a_akgun','tle_gitlab','dstull','imphill']
    ) WITH ORDINALITY AS t(username, ord) USING(username)
WHERE "project_authorizations"."project_id" = 278964
    AND ("users"."state" IN ('active'))
    AND (
        "users"."user_type" IS NULL
        OR "users"."user_type" IN (6, 4)
    )
ORDER BY t.ord

Plan

Time: 96.376 ms  
  - planning: 10.116 ms  
  - execution: 86.260 ms  
    - I/O read: 85.532 ms  
    - I/O write: 0.000 ms  
  
Shared buffers:  
  - hits: 12 (~96.00 KiB) from the buffer pool  
  - reads: 23 (~184.00 KiB) from the OS file cache, including disk I/O  
  - dirtied: 0  
  - writes: 0  

https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/12494/commands/44139

How to set up and validate locally

1. Ensure a SaaS (Gitlab.com) environment
   1. One way of doing this is to add a env.runit file to the root GDK folder with the following snippet

      export GITLAB_SIMULATE_SAAS=1

2. Set ultimate license on a group http://gdk.test:3000/admin/groups
3. Create a project in the ultimate group or use an existing one, e.g. http://gdk.test:3000/gitlab-org/gitlab-test
4. Set the feature flag on rails console bundle exec rails c

   project = Project.find(2)
   Feature.enable(:suggested_reviewers_control, project)

5. Enable suggested_reviewers project settings

   project.project_setting.update!(suggested_reviewers_enabled: true)

6. Create a merge request on the project
7. Use some existing project members, make sure to select the usernames in a reverse order

   project.authorized_users.map(&:username)
   => ["root", "erinn_stoltenberg", "lorenza", "lea_mclaughlin", "curtis.tromp", "ken", "shayne.armstrong"]
   suggested_reviewers = ["shayne.armstrong", "curtis.tromp"]

8. Add some suggestions on rails console bundle exec rails console

   mr = MergeRequest.find(7)
   mr.build_predictions
   mr.predictions.update!(suggested_reviewers: { reviewers: suggested_reviewers })

9. Confirm via the console and UI that the suggested reviewers display in the correct order

   mr.suggested_reviewer_users
   => [#<User id:17 @shayne.armstrong>, #<User id:10 @curtis.tromp>]

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #376024 (closed)