Fix Dask Dashboard link (#260)

Author: jacobtomlinson

binder/start

@@ -2,6 +2,7 @@
 
 # Replace DASK_DASHBOARD_URL with the proxy location
 sed -i -e "s|DASK_DASHBOARD_URL|/user/${JUPYTERHUB_USER}/proxy/8787|g" binder/jupyterlab-workspace.json
+export DASK_DISTRIBUTED__DASHBOARD__LINK="${JUPYTERHUB_SERVICE_PREFIX}proxy/{port}/status"
 
 # Import the workspace
 jupyter lab workspaces import binder/jupyterlab-workspace.json