Renaming "Fund" model to "Account"

Changes the "Fund" model to "Account" to make it clear that the model only references the money, and not all the other things that go with a particular fund (which we store in Campaign or Project, depending on the fund type).

Sorry, I know this changes basically every file. But @CM and I think we have the models in stable shape after this change, so we shouldn't (famous last words) need to do any more of this kind of refactoring.