Presence data, and report presence

[gnprice]

See subsystem doc:
https://zulip.readthedocs.io/en/latest/subsystems/presence.html
and API docs:
https://zulip.com/api/get-events#presence
https://zulip.com/api/get-presence

This is one of the handful of areas where the Zulip API remains underdocumented, so see also the corresponding code in zulip-mobile, and the references linked from zulip/zulip#13519 .

Once we're tracking presence data, it should appear in the UI:

• Show presence (active/inactive) in the app #1607

Related issues

• Support toggling "Invisible mode" #1578