Approval refactoring

* Added validation on the `ApprovalGroup#status`
* Fewer instance variables
* More simplified/unified code path for going from controller to mailer when approving
Yep, it's gone. I'll add a story to fix this.

![screen shot 2014-11-07 at 1 18 08 pm](https://cloud.githubusercontent.com/assets/11333/4958873/355d930c-66b3-11e4-8218-ba19224f581f.png)