export/import media file captions

test plan:
 - have a course with a video file embedded in content somewhere
   (such as a wiki page)
 - create captions for the video and verify that they play back
 - export this course to an .imscc
 - import it to a new course
 - ensure the captions play back in the copied course

fixes CNVS-10228

Change-Id: Ie15076bc31ad36efabf7a718d12bd8a9b12e45e1
Reviewed-on: https://gerrit.instructure.com/32680
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>