Modify the dask dashboard to work through the yarn proxy

The dask dashboard relies on modern web features like websockets, which are incompatible with the yarn proxy. To get around this we can either:

- Modify the dashboard to optionally use ajax polling, as seen in [this example](https://github.com/jcrist/skein/tree/master/examples/pricing_dashboard)
- Create a new, simpler dashboard to use with yarn deployments. The dashboard is just a scheduler plugin, so it wouldn't necessarily have to live in the distributed repo (although it might).