Put transition date on analytics graph

* Show a line on the analytics graph on the date at which the site mappings went live
* Only show a line when site is live and the graph spans the date of transition

![screen shot 2014-04-17 at 16 15 29](https://cloud.githubusercontent.com/assets/319055/2734864/4ace2814-c659-11e3-9fac-18a61a42bb74.png)
![screen shot 2014-04-17 at 16 15 43](https://cloud.githubusercontent.com/assets/319055/2734863/4acdb3ca-c659-11e3-831f-05c24350bc1b.png)