Refactor views

This adds a Subject view and moves some of the logic over from  NotificationDetails view. This is to prepare for more features around comments including being able to create comments and show review comments on pull requests.

It also adds the concept of a "banner" that is shown at the top of the comments.

![github_notifications](https://f.cloud.github.com/assets/173/1806877/b14be9fe-6cc3-11e3-844e-68779ac4a66f.png)