improved accessibility of tag cloud

This makes it easier for screen readers to use the tag cloud (and associated sort buttons).

The only thing that is visible for non-screen reader users is that the sorting appears above the tag cloud instead of below. Screen reader users didn't see the sort options in tests when it was below.
![tag-cloud](https://cloud.githubusercontent.com/assets/74879/3897977/b98b207c-2266-11e4-9798-f88af5717b1c.png)