Use workload + breakdown in dashboard

This adds pending count to dashboard assignment navigation (left sidebar).

It doesn't yet account for the logic around no nits ever vs no nits this iteration.

These are based on Workload and Breakdown which move a lot of the underlying dashboard logic into the model layer. They've got some very fat integration tests.

https://github.com/kytrinyx/exercism.io/commits/master/lib/exercism/breakdown.rb
https://github.com/kytrinyx/exercism.io/blob/master/lib/exercism/workload.rb

There's still a fair amount of cleaning up to be done in the view code, and (in particular) the view helper code.