Monitor remaining available disk for spilling

[hendrikmakait]

It would be helpful to see how much disk is available for further spilling on the /workers dashboard. This way, the user can gauge the utilization of the disk and whether it will run out of space soon.

In coiled/benchmarks#229, it is virtually impossible to understand when a machine will run out of space from the information available on the dashboard and the user has to dig through worker logs if a worker disappears unexpectedly.