When there is an assignee list and the board can show a lot of issues,
the Open list can be very slow. This was because the query was
essentially repeated inside a NOT IN to exclude issues that are assigned
to someone from one of the assignee lists.

Instead, we can simplify the inner query to be a more direct one:

    NOT EXISTS (
      SELECT 1
      FROM issue_assignees
      WHERE user_id IN ($assignee_list_users)
        AND issue_id = issues.id
    )

Which also performs much better, as well as being simpler to understand.