FEATURE: show raw email for replies/topics created via email

Demo:

![raw_email_modal](https://cloud.githubusercontent.com/assets/5732281/4682193/5b950324-561a-11e4-8559-c960e39fb221.gif)

@coding-horror I want a nobel prize too! :smile: 
Not sure we want to **always** serialize `raw_email`. This is only intended for staff members and should be requested _on the fly_ when you click on the envelope icon.