Support muted_topics format for pre-Zulip Server 6

[gnprice]

In my initial implementation of muted streams and topics #346, I intend to leave out support for the old muted_topics format. It's not fundamentally hard to do, but it'll be a bit annoying.

The impact of this is that when using Zulip Server 5.x and older, the client won't be aware of any muted topics. (There won't be any unmuted or followed topics in any case, because those servers don't have those features.)

Zulip Server 6 was released 2022-11-17, just over a year ago. So we're a few months away from dropping support for 5.x (in a successor issue to #268). If we approach a full production launch before that time, then we'll come back and deal with this.

[gnprice]

We're not going to implement this at this point — we'll drop support for those now-ancient server versions instead.