Support logged-out viewing of Zulip content

[gnprice]

Some Zulip organizations allow people to read some of their content without logging in. Our docs often call this the "public access option":
https://zulip.com/help/public-access-option

The Zulip web app supports operating in this mode. It'd be neat to offer the same in the mobile app.

This will be a major project. Lots of places in the app's code will need to change to accommodate the possibility that the user isn't logged in. Consequences of that situation include (a) there is no self-user; (b) the data given to us by the server is different and sparser in all the ways that happen when not logged in; (c) the user isn't able to do things like send messages, mark read/unread, etc., and shouldn't be presented with UI for doing so.

[gnprice]

Requested in chat last week: https://chat.zulip.org/#narrow/channel/48-mobile/topic/mobile.20beta.20feedback/near/1989116