Bulk captions download subs bug fix

Fixes a bug in handling bug captions where the download captions button didn't
appear on a video's view even with the option to allow caption downloads set to
true and captions properly set through the bulk captions utiltity.
The button would not appear until the studio user saved the video settings
again.

The cause of the bug was within the operation of the bulk captions
utility downloading the subtitles for a video from youtube. It correctly
downloaded and saved the subs to the database, but never set the sub field
of the video descriptor to be the id of the new subtitles saved in the
db. To fix this, a pre-existing function which handles all needed updates
is used to ensure that the sub field is properly updated along with the subs
saved in the db.
Merge pull request #145 from Stanford-Online/njdup/bulkcaptions_fix

Bulk captions download subs bug fix