material property vs action parameter: Val fields are inconsistent

[miketynes]

vw_material_property expects a val_val, but action parameter just a val

[miketynes]

Question from @vshekar: do we want property_x to have a generic foreign key?
Potential answer: no -- having one property_x minimizes effort from a psql implementation perspective but not from a django perspective. aside from minimizing effort, we cant think of any advantages

[miketynes]

https://stackoverflow.com/questions/54217937/how-to-bind-multiple-models-to-one-manytomanyfield

[miketynes]

New issue related to shekar's question: #108