Add placeholders for text input on Extensions dash feature

I noticed this while reviewing the recent PRs from @bradenmacdonald. This PR adds in placeholders on the text input fields which increases accessibility:
![screen shot 2014-08-20 at 12 02 30 pm](https://cloud.githubusercontent.com/assets/1985317/3983917/a3b51f50-2883-11e4-8fb9-b808166d746a.png)

The text and styling is the same as other places on the instructor dashboard:
![screen shot 2014-08-20 at 12 02 42 pm](https://cloud.githubusercontent.com/assets/1985317/3983922/ae184012-2883-11e4-99b6-be16196e67d7.png)

@bradenmacdonald  and @carsongee would you mind taking a look?