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
Assets Page Template Update

For embed vs. external URLs

STUD-1338
removed dup'd paragraph
Add test for external_url being passed to assets page.
Merge pull request #2579 from edx/giulio/studio-file-urls

Internal/External Studio file URLs