Handle `<time>` elements ("global times")

We should handle `<time>` elements. These are the output of the Zulip "global times" feature: https://zulip.com/help/global-times

Handling these means parsing them in `lib/model/content.dart`, and then styling them appropriately in `lib/widgets/content.dart`. For appropriate styling, see the Zulip web app's `web/styles/rendered_markdown.css`.

