Use max_topic_length instead of hardcoded limit of 60

[gnprice]

Until recently, the Zulip API specified a fixed maximum length for topics, of 60 characters. This changed a few months ago in zulip/zulip@e68a69e / zulip/zulip#25226 ; the maximum is now taken from the register-queue response, as max_topic_length.

(That value had actually existed in the register-queue response for a few years, but wasn't part of the documented behavior of the send-message endpoint.)

We currently have a constant kMaxTopicLength reflecting the old API. We should instead use max_topic_length.

Chat discussion:
https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/flutter.20app.20and.20max_topic_length/near/1643895

[kehshiba]

@gnprice can i work on this?

[gnprice]

@kehshiba Hello! Please take a look at the Zulip project's guidance for picking an issue to work on.

In this repo we don't have the "help wanted" label because this codebase isn't yet fully set up to welcome contributors; but that just makes the other steps all the more important.

[kehshiba]

ok thanks @gnprice