Meanwhile, anybody who follows the news is familiar with the flight of Edward Snowden, who is arguably the most important whistleblower in American history for his massive leak of secret NSA documents. Even so, the Danish-made film “Chasing Edward Snowden” about the details of his risky escape from the U.S. to avoid arrest — and, some have contended, possible assassination — tells the story with an authority, clarity, and drama that make it too gripping to miss. It was recently posted on YouTube.