Disable parts of the timeline as the dates for them pass

As each date in the timeline passes, it will show as "disabled" to show that that item has passed. I've styled it as such:

![](https://cloud.githubusercontent.com/assets/494732/3321681/d692bfa6-f73a-11e3-8385-6b9bf4a47e23.png)

There's a lot of duplicated logic here, so it would be nice to have this whole timeline item in a helper, e.g. `@Html.TimelineItem(periodPassed, periodStartDate, label)`.