Approval refactoring

* Added validation on the `ApprovalGroup#status`
* Fewer instance variables
* More simplified/unified code path for going from controller to mailer when approving
If I'm understanding correctly, it will never be `#blank?`:

https://github.com/18F/C2/blob/0a271c61c9a5fd296030638bbc8c935ed408e6c8/app/decorators/cart_decorator.rb#L34-L40