Add static fallback method for video transcripts

Courses that are imported with the --nostatic flag do not show transcripts/captions properly even if those captions are stored inside their static folder.  This adds a last resort method of redirecting to the static asset path of the course if the transcript can't be found inside the contentstore and the course has the static_asset_path field set.

One thing I thought was strange is that value of static_asset_path didn't seem to be inherited down even though it is inheritance.py and is available on the module but set with the default value instead of the value at the course descriptor, so I had to get the course descriptor to retrieve the proper value. If there is a better method, I would be interested in changing to it instead.

This is only my second journey into xmodules/modulestore @sarina so I'm still not sure on reviewers.

Heads up to @pdpinch and @ichuang