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.