Hide date controls from schedule screenshots

... so they look like this: 

![image](https://f.cloud.github.com/assets/159687/2340195/b051100e-a4bf-11e3-8334-29000241a7e2.png)

Also add a button to the `/admin/backfill_schedules` page to kick-off a backfill.

![image](https://f.cloud.github.com/assets/159687/2340199/b6b65558-a4bf-11e3-9abc-c988ce6b5280.png)