Rework users#update and add auto save role change

@jwswj I want to merge this into your routing PR

![screenshot](https://cloud.githubusercontent.com/assets/1329716/3182958/52536484-ec5e-11e3-823a-542616907be5.gif)
![screenshot](https://cloud.githubusercontent.com/assets/1329716/3182959/61fec342-ec5e-11e3-8070-939cd80d9dde.gif)

/cc @zendesk/samson