KRACOEUS-7308: JPA work for budget

Just want to check if this is really the correct name for this class. While I see that this is the base class for Budget and BudgetVersionOverview, we are going to need a ProposalBudget and AwardBudget at some point. Is Budget going to become a MappedSuperClass as well? Or do want to rethink the naming and maybe structure now?