Discussion hyperlink button changes

Updates to the hyperlink button. 

The button now says "link" as opposed to "http://" & the logic has changed to more closely remember R2. 

The user is now prompted input the link URL if one isn't provided, as they would in R2.

![screen shot 2014-10-09 at 17 27 44](https://cloud.githubusercontent.com/assets/2236852/4579937/42f5159c-4fd1-11e4-8595-fff8e04d741b.png)



In Javascript you can write this conditional in one line as: `selectedText = selectedText || linkURL;`