Use SubtitleLoader to load our subtitles

This method allows us to store the styling/layout data in the amara
repo instead of a string in the babelsubs repo.  I also switched the
styles to use the suggested ones from #1012.

Also changed the DFXP that we create for new subtitles in the
javascript editor.
Fixed missing attribute (#1507)
Fix for downloading all subs

Made the code use our SubtitleLoader so that we get the custom TTML
header.
Removed xml:lang attribute from our content div

There's no need to do that and we don't do it for subsequent divs
created by the new paragraph button.
Don't show download all link if it's not needed

If there are 0 or 1 public versions, then it doesn't make sense to
show the download all link.
Fixed import error in unittests
Merge pull request #1510 from pculture/brightcove

Brightcove