Add search for merge_user in merge request view and API

What does this MR do and why?

Implementation of #384574 (closed). This way it is possible to filter merged MRs by the person merging it. Also supports negation.

Screenshots or screen recordings

How to set up and validate locally

1. Set-up a local instance
2. chose any project, that has merged MRs (I tested with gitlab-org/gitlab-shell)
3. execute searches

Changes affecting DB Queries

find user by name or id

The feature uses the already existing User.find_by_id or User.find_by_username. One of these is executed per request, to create a user object for searching.

-- find_by_username
SELECT
    users.*
FROM
    users
WHERE
    lower( users.username ) IN ( lower( 'root' ) )
LIMIT 1;

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/24906/commands/79094

-- find_by_id
SELECT
    users.*
FROM
    users
WHERE
    users.id = 20
LIMIT 1;

Filter by existing user

The feature extends the existing queries for the merge_request dashboard by an additional feature. The main queries for the dashboard are extended by clauses for merge_request_metrics.merged_by_id, everything else is pre-exiting. Resulting queries without additional filters are:

-- filter by existing user
SELECT
    count(*) AS count_all,
    merge_requests.state_id AS merge_requests_state_id
FROM
    merge_requests
    JOIN merge_request_metrics ON merge_request_metrics.merge_request_id = merge_requests.id
WHERE
    merge_request_metrics.target_project_id = 3 AND
    merge_request_metrics.merged_by_id = 20
GROUP BY
    merge_requests.state_id;

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/24990/commands/79396

SELECT
    merge_requests.*
FROM
    merge_requests
    JOIN merge_request_metrics ON merge_request_metrics.merge_request_id = merge_requests.id
WHERE
    merge_requests.state_id IN ( 3 ) AND
    merge_request_metrics.target_project_id = 3 AND
    merge_request_metrics.merged_by_id = 20
ORDER BY
    merge_requests.created_at DESC,
    merge_requests.id DESC
LIMIT 20
OFFSET 0;

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/24990/commands/79397

Filter by non-existing user

-- filter by non-existing user

-- no query emmitted

-- no query emmitted

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.