Fix cross join when filtering by assignee

What does this MR do and why?

This changes filtering by assignee usernames, so that we pluck the user ids separately instead of using a subquery.

Query changes

Before

SELECT COUNT(*), COALESCE(SUM(weight), 0)
FROM "issues"
WHERE "issues"."id" IN (
  SELECT "issues"."id"
  FROM "issues"
    INNER JOIN "work_item_types" ON "work_item_types"."id" = "issues"."work_item_type_id"
  WHERE (NOT EXISTS (SELECT 1 FROM "banned_users" WHERE (issues.author_id = banned_users.user_id)))
    AND "issues"."project_id" = 1
    AND ("issues"."state_id" IN (1))
    AND EXISTS (
      SELECT "issue_assignees".* FROM "issue_assignees" WHERE "issue_assignees"."user_id" IN (
        SELECT "users"."id" FROM "users" WHERE (LOWER("users"."username") IN (LOWER('user1'), LOWER('user3')))
      )
      AND (issue_id = issues.id)
    )
    AND "work_item_types"."base_type" IN (0, 1)
    AND (EXISTS (SELECT "label_links".* FROM "label_links" WHERE "label_links"."target_type" = 'Issue' AND ("issues"."id" = "label_links"."target_id") AND "label_links"."label_id" = 1 LIMIT 1))
)

https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/20552/commands/67397

After

The users query is now done separately so we'll have:

SELECT "users"."id" FROM "users" WHERE (LOWER("users"."username") IN (LOWER('user1'), LOWER('user3')))

https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/20552/commands/67399

And then the result of this inside the WHERE "issue_assignees"."user_id" IN (...) clause:

https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/20552/commands/67401

MR acceptance checklist

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

Related to #417465 (closed)