To increase portability of courses, have the Asset Index page display the classic /static/... URL shorthand.

which is more portable across course runs

Note, unfortunately our unit tests do not support File Upload testing. Per @jzoldak this is a known issue that has not yet been resolved. I'll write some unit tests again importing a course from XML and then verifying the Asset Index page returns the expected portable_url.

@cahrens @dmitchell can you get started on the review.