Show topics on channels page

[gnprice]

On the screen showing the list of your subscribed channels/streams (from #187), it'd be useful to also have a feature to show the topics within each channel, by tapping something to expand a channel — akin to web's left sidebar.

This doesn't exist in the zulip-mobile RN app, so it could be a post-launch feature. On the other hand, zulip-mobile does have a dedicated screen for listing the topics in a channel, which serves an overlapping set of use cases; it's possible we could leave out that feature if we build this one instead.

[chrisbobbe]

We've had a request for this in beta feedback: https://chat.zulip.org/#narrow/channel/48-mobile/topic/mobile.20beta.20feedback/near/2002291

How, where I can acces list of Topics in the Channel in the new Flutter app?

[gnprice]

Separately this was requested today:

Channels. I think this is a reasonable default, but Channels on mobile doesn't organize by topic - and by-topic is now the default for Desktop (it is quite hard to get a view of a channel as a feed). I am not sure whether that was a UX mistake - would want more research - but regardless if you want to stick with it, the mobile app should behave the same way.

along with including it as one of the home-view options in #517.

(I think the other request mentioned above would probably be more fully satisfied by #1158.)

[gnprice]

Feedback this week:

In my view, an accordion presentation of the topics inside their channels, similar to the presentation on the web app, would be the biggest singular improvement of the mobile UX, because this presentation matches the data model (channel -> topic -> message). Someone who is not aware of that model and gets to know Zulip through the exclusive use of the mobile app would immediately pick it up, something they are unlikely to do on the RN app, in my opinion.