Show unread markers in message list

[gnprice]

This means:

• Track which messages are read vs. unread.
• When a message in the message list is unread, show that with an unread-message marker (https://zulip.com/help/marking-messages-as-read).
• When a message that was unread becomes read, remove the marker with a fading animation, just like Zulip web and zulip-mobile.

To make the UX fully hang together, we'll follow up with:

• Open message list at first unread, rather than latest message #80
• Mark messages as read on scroll, configurably #81