Render “Staged” in task list with yellow health badge

This was the intended visual effect for the task list, but it was lost
in an earlier refactoring

* Extract the logic of a health badge into a BadgeComponent. If `type`
  is provided, the badge gets a gray badge and a health indicator.

The gray badge and yellow indicator are added with this PR. Previously
the staged status got no special visual treatment:
![screenshot 2014-07-02 16 16 17](https://cloud.githubusercontent.com/assets/29612/3463923/e7429138-0242-11e4-9423-45c2d7fc8fba.png)