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
Log API Errors from youtube
Merge branch 'syncing-triggers' into staging
Fixed JS syntax error (#1637)
Changes to syncing triggers

The old method was to sync if subtitles_complete changed or a new
version was added.  This was problematic because in
pipeline.add_subtitles() we might want to change subtitles_complete
and then add a new version as one atomic action.  This would mess with
what we synced.

Changed it so that after add_subtitles() we emit the subtitles_changed
signal.  The externalsites code connects to that and decides if we
should sync or not.

Also if subtitles_complete gets changed outside of add_subtitles(), we
will send the subtitles_changed signal then.
Merge pull request #1642 from pculture/syncing-triggers2

Changes to syncing triggers - looks all good to me
Add the credit test even if subtitles are not complete

I believe this matches the old behavior, but in any case it makes
sense.  The text is a request to subtitle the video on amara.
So even if there are not subtitles yet, having the link still makes
sense.  Now that I think about it "credit" is really the best word for
this.
For youtube videos sync to channels not owned by the amara owner (#1643)

This makes it so that if you link you're youtube account, then all
youtube videos for that account will be synced.  It doesn't matter if
you are the owner for the amara video or not.
Show the sync history tab for non-team videos

We still only show it for superusers though.
Honor send_subtitles_changed when saving SubtitleLanguages

Oops, not sure how I missed this one originally.
Don't re-sync imported subtitles (#1646)
Merge pull request #1648 from pculture/gh-1643

Gh 1643
Merge branch 'staging' into dev

Conflicts:
	apps/externalsites/signalhandlers.py
	apps/externalsites/tests/syncing.py
	apps/subtitles/models.py
	apps/subtitles/pipeline.py
	apps/subtitles/tests/signals.py