Return dispositions in a form closer to what's defined in the MDES.

Reasons:

1. Cases' Event model can give us an NcsNavigator::Mdes::DispositionCode
   object that is much richer than a tuple of two numbers.  We might as
   well take advantage of that.
2. If we do (1), then Pancakes does not have to do any indirection to
   display a disposition fit for human consumption.