Add search for merge_user in merge request view and API

What does this MR do and why?

Implementation of #384574. 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.