Ensure the sortable handle always has enough space

For https://github.com/gregbell/active_admin/pull/2656 (please do a command line merge of this so we don't end up with a merge commit)

These changes get you:
![screen shot 2013-11-17 at 12 12 04 am](https://f.cloud.github.com/assets/688886/1557925/4368ac9c-4f4f-11e3-9f4f-f3f25e913945.png)