move some responsive bootstrap transitions

- `-md-` puts the transition at 970px
- `-sm-` puts the transition at 768px

I moved most of the `md` transitions to `sm`,
which seems more reasonable.
I also moved the cluster list to `xs` because
the columns will never be wider than a dozen or so characters.
Merge pull request #6204 from minrk/xs-cluster-columns

move some responsive bootstrap transitions