Validate new url in whitelist admin UI

For discussion. Needs a little thinking about whether allowing all admins to add to the whitelist is appropriate - perhaps it should be a distinct permission.

![screen shot 2014-05-14 at 14 35 02](https://cloud.githubusercontent.com/assets/93739/2971643/98eff536-db6c-11e3-8819-0f0dc58ad250.png)