Handle video previews

[gnprice]

We should handle video preview elements in message content. Here's an example message that uses one:
https://chat.zulip.org/#narrow/stream/9-issues/topic/.E2.9C.94.20inbox.20unread.20dot.20only.20present.20in.20view/near/1668976

To get the server's HTML for that message, see comments in test/model/content_test.dart. (Don't try to get the HTML from the web app via browser devtools; this is one kind of content where the web app is known to show HTML that's been adjusted from what the server gave it.)

Handling these means parsing them in lib/model/content.dart, and then displaying them appropriately in lib/widgets/content.dart.

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In an ideal version of this feature,

• we'd start by showing a still thumbnail image, probably with a "this is a video" indication similar to what web has;
• then if the user taps on the video, we'd play it, probably after going to the lightbox;
• and when playing, we'd have appropriate controls to pause, to resume, to start over, etc.

If we can readily get all of that functionality off the shelf, then great. If not, then for an initial version it'd be good enough to:

• start by showing a still thumbnail image, with a "this is a video" indication as above;
• then if the user taps on the video, send them to a web browser with a link that will let them view it.

In that case we'd file a followup issue for the remaining functionality.