fix layout of external storage config table

This fixes the trash bin icon and the user input box.

cc @owncloud/designers @icewind1991 

Broken:

![external-broken](https://cloud.githubusercontent.com/assets/245432/2801783/d607ce02-cc86-11e3-84cf-8bf79b7e21b0.png)

Fixed:

![external-fixed](https://cloud.githubusercontent.com/assets/245432/2801787/d994b922-cc86-11e3-9544-e8fb4ad0a249.png)