Handle inline audio previews

[gnprice]

This is an upcoming Zulip feature.

This issue is for a basic version that just shows a link the user can use to get to the file on the web. A follow-up issue #1666 is for showing an actual audio player.

References

• #api design > audio previews @ 💬
• Server PR: markdown: Render inline audio files using the ![title](url) syntax. zulip#34903
• Follow-up issue: Audio player for inline audio in message content #1666