Changing .push-{left,right}  to .navigate-{left,right}

We were using `.push-left` and `push-right` to add chevrons to elements. This works fine but it's confusing considering we use `.pull-left` and `.pull-right` for floating objects. They seem like they'd be related but they're not.

This branch uses `.navigate-left` and `.navigate-right` instead.

Fixes #338.