prompt visitor to authenticate on clicking dummy comment field on logged out discussion view

@jonlemmon this is a pretty cheap solution, what do you think?

![screen shot 2014-02-09 at 2 35 03 pm](https://f.cloud.github.com/assets/970124/2118884/b9d1c4d2-912a-11e3-9786-bad9ec889160.png)
![screen shot 2014-02-09 at 2 34 48 pm](https://f.cloud.github.com/assets/970124/2118886/dea0abd4-912a-11e3-8d5e-668715bc01af.png)