Refactor ManageRafflesVM for UoW first pass

ManageRafflesViewModel:
* Removed most direct calls to the db context.
* Hooked up Read, Update, Delete and Add operations through UoW.

ViewModelLocator:
* Updated to pass in the UoW into the ManageRafflesViewModel.

RaffleModel:
* Minor update so that the Winners property will initialize if the
entity is null.