Use workload + breakdown in dashboard

This adds pending count to dashboard assignment navigation (left sidebar).

It adds `wants_opinions` to the featured pages.
It also pulls "looks great" submissions into the featured list for locksmiths.

It does not yet account for the logic around no nits ever vs no nits this iteration.

The Dashboard presenter has been entirely rewritten using the Workload and Breakdown classes which move a lot of the underlying dashboard logic into the model layer. Those model objects have some very fat integration tests.

There's still a fair amount of cleaning up to be done in the view code, and (in particular) the view helper code.