Rely on Zulip Server 7 APIs

[gnprice]

Since #1521, the app refuses to connect to servers older than Zulip Server 7. That means we can simplify away all the spots in our codebase where we have logic to accommodate older versions of the Zulip server.

A prerequisite issue #1837 is for relying on server 6, as marked by comments TODO(server-6). This one is for relying on server 7, as marked by comments TODO(server-7). For details, see #1837, #268, and the previous issues linked from the latter.

References

• Preceding round of the same thing (a prerequisite): Rely on Zulip Server 6 APIs #1837
• PR that disallowed connecting to older servers (a prerequisite for this issue): api: Like zulip-mobile, don't allow connecting to Zulip Server <7.0 #1521