Preview inline videos directly in message list

[gnprice]

Since #587, we show videos in messages. We don't show them in the actual message list, though:

• For an "embed" video (meaning it's on some external service, which Zulip web would embed in an iframe), we show a thumbnail, and tapping it opens the video as a link outside Zulip.
• For an "inline" video (meaning the actual video is hosted on the Zulip server or somewhere it trusts), we show just a black box with a "play" icon, but tapping it opens the video directly in the app's own lightbox.

For embed videos, what we have is about as good as we can get. But for inline videos, we can in principle do better still:

• First, it'd be good to show some kind of thumbnail or still frame from the video. These often help people see at a glance some idea of what the video is and decide if they want to look further.
• Fancier than that, it'd be neat to buffer the beginning of the video when the message scrolls into view, before the user interacts with it. That way if they do decide to play it, the video can start immediately with no lag.

Early revisions of #587 had a version of this functionality, but it got removed before merge. For details, see:

• The original PR description: content: Handle video previews #587 (comment)

  This includes mention of a couple of bugs which we'd need to deal with, affecting package:video_player on iOS.

• More discussion of those bugs:
  content: Handle video previews #587 (comment)
  content: Handle video previews #587 (review)
  content: Handle video previews #587 (comment)
  content: Handle video previews #587 (comment)