move phlPreK and paSeniorFoodBox models into benefits.dmn

[prestoncabe]

Why?

• I think it is more elegant to call endpoints like /benefits/phlPreK rather than /phlPreK/results (fits nicely with /benefits/all)
• I think it might be better to organize benefit models within one DMN file (with liberal use of separate DRDs for grouping things - like all benefits from a particular state in one DRD for example), since it is impossible with the current DMN editor (apparently) to have DMNs that reference each other from different directories (and I want to keep that directory from getting too unwieldy).

Note: should perhaps do this in tandem with #17