Move filtering from API::Groups#find_groups into GroupsFinder

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Move filtering and ordering done on https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/api/groups.rb#L30 into GroupProjectsFinder.

The following statements should me moved into GroupFinder

        groups = groups.search(params[:search]) if params[:search].present?
        groups = groups.where.not(id: params[:skip_groups]) if params[:skip_groups].present?
        order_options = { params[:order_by] => params[:sort] }
        order_options["id"] ||= "asc"
        groups = groups.reorder(order_options)