Show "typing" status

[gnprice]

I.e., half of this feature:
https://zulip.com/help/typing-notifications
along with #666.

In the API, the relevant data appears as events:
https://zulip.com/api/get-events#typing-start
https://zulip.com/api/get-events#typing-stop

In the code, let's organize the data under a class TypingStatus, similar to Unreads or RecentDmConversationsView.

For logic for comparison, see what's in zulip-mobile or Zulip web. The implementation in zulip-mobile isn't great in how it handles expiry, though (in typingActions.js) — in particular we shouldn't need a polling loop. I haven't studied the web implementation.