Update when we sync subtitles to external sites

Before we would sync when the public tip changed.  One issue with this
was that we would save subtitles when users clicked the save button,
which we really mean for saving a draft.

Changed it so that we sync when:
  - A new version is added and the subtitles are marked complete
  - The subtitles_complete filed changes to True
Merge branch 'syncing-triggers' into staging