Fix subtitle actions with tasks

Added TaskSaveDraft action, which correctly sets subtitle_visibility
to private when initially creating the version.

Use Publish() action for post-publish edits.
Fix webdriver test

Changed it so we don't update the video title when title=''.  I think
that the API user should always send a title, but it seems OK to still
have the check.

Updated webdriver test to check for the updated speaker name.