move some responsive bootstrap transitions

ref: [bootstrap grid media queries](http://getbootstrap.com/css/#grid-media-queries)

- `-md-` puts the transition at 970px, which is pretty wide.
- `-sm-` puts the transition at 768px

I moved most of the `md` transitions to `sm`, which seems more reasonable in most cases. I also moved the cluster list to `xs` because the columns will never be wider than a dozen or so characters.

before:

![screen shot 2014-07-24 at 13 03 48](https://cloud.githubusercontent.com/assets/151929/3693986/c60bd84e-136d-11e4-80cd-4b9fd1cd0677.PNG)

after:

![screen shot 2014-07-24 at 13 04 09](https://cloud.githubusercontent.com/assets/151929/3693989/cb832700-136d-11e4-8636-1088f4c53772.PNG)