Add "new campaign" shortcut

An attempt to make it easier to start a new campaign from a fresh initialization of the app, this adds a shortcut to "new campaign" in the admin navbar partial.

![screen shot 2013-11-18 at 5 09 37 pm](https://f.cloud.github.com/assets/1132438/1568771/48b0ffd8-50b7-11e3-9e75-d6a238253f80.png)