Internal/External Studio file URLs

- Changed link URL of file name to use public/LMS url
- Added toggle link to display all public URLs for instructor convenience

On page load, the page looks pretty much identical to before.

![screen shot 2014-02-12 at 11 09 14 am](https://f.cloud.github.com/assets/3364609/2152447/3792dc58-9419-11e3-9448-694fcbd69bcf.png)


When you click the new "Show external URLs" toggle link, all of the asset URL readonly text fields switch to displaying external URLs.

![screen shot 2014-02-12 at 11 09 20 am](https://f.cloud.github.com/assets/3364609/2152448/3a1214e4-9419-11e3-82ed-45bde80ad245.png)