Notify user if the server version is unsupported

[preetmishra]

With the recent discussions in #653 and #zulip-terminal > Supporting multiple server versions, we plan to support server version 2.1.0 and above (see README.md and versions.py) officially.

Consequently, we would like our users to be notified, in some way, if their version is not expected to be supported.

We've had some discussion about how we would actually want that to happen in #zulip-terminal > Supporting multiple server versions (e.g: by adding '(Unsupported)' suffix to the server version in AboutView, by showing a warning in NoticeView, by showing a warning at the startup, etc).

We encourage you to drop your ideas here or in #zulip-terminal > Supporting multiple server versions before claiming the issue.