UI for topic muting/unmuting/following

[gnprice]

This will be a followup from #346, and a complement to #347.

In zulip-mobile, we offer this only for muting and unmuting — we haven't implemented the new "follow topic" feature. At a minimum, for launch, we should offer the same level of functionality as zulip-mobile; but then probably one of the first features we'd want to add after launch would be full "follow topic" support, so we might build the full version in the first place.

In zulip-mobile, we offer this in one place, the "topic action sheet", which you get by long-pressing a topic:

• as the app bar of a topic narrow;
• as the recipient header of a stream message in the message list;
• in the screen with the list of topics for a stream;
• in the inbox view.

We should do something broadly comparable, unless we do leap ahead to include "follow topic" support. We might in any case vary the details; for example if you long-press the stream name within a recipient header, that should perhaps give the stream action sheet instead of the topic action sheet.

[PIG208]

Topic following in zulip-mobile was added on Nov 22, 2023. To offer the same level of functionality, we should probably implement following as well.

I think a design requirement of this feature is to also display mute/following states for each topic. At this point we can borrow that from the legacy mobile app.