Track user status

[gnprice]

This Zulip feature: https://zulip.com/help/status-and-availability
but only for tracking the data; setting one's own status is a separate issue #198.

In the API this is:
https://zulip.com/api/get-events#user_status
and search for "user_status" on https://zulip.com/api/register-queue .

This information should then appear in the UI at:

• ui: Add list of recent DM conversations #119
• Profile screen for other users #195
• and possibly other places.