missing jQuery UI image assets

Firebug reports

    404 Not Found /assets/images/ui-bg_inset-hard_100_fcfdfd_1x100.png
    404 Not Found /assets/images/ui-bg_glass_75_d0e5f5_1x400.png

This is most likely because image references in `vendor/assets/stylesheets/jquery-ui-*.custom.css` do not use the asset pipeline (i.e. `url("<%= asset_data_uri(...) %>")`).