Hide the appropriate reorder buttons on the first and last app in the list of selected apps in the multiple-app selector.

Janky selectors, open to suggestions. Tried to get `:first-of-type` and `:last-of-type` to work. They didn't.

![screen shot 2014-07-22 at 2 36 55 pm](https://cloud.githubusercontent.com/assets/23885/3663925/e68f0dda-11d7-11e4-9fb8-655fe27de818.png)

@ngokevin FYI it's actually a little faster to do `cursor` on `element` over `element:hover` since the rendering engine doesn't have to recalculate the new style on hover.