List of topics in channel

[gnprice]

In the legacy app, if you're looking at the messages in a topic, we have a button in the app bar (an "up" icon) that gets you a list of all the topics in that channel. It's sorted by recency, and shows the number of unreads, whether you have an @-mention, and whether the topic is muted etc.

Basically it's similar to what you get in the web left sidebar after hitting "Show all topics" under a given channel.

We should offer the same thing in the new app. I wasn't sure people really use that feature; but it came up in user feedback after yesterday's beta announcement, so that's a sign that people do.

Related issue:

• A different feature serving some of the same use cases: Show topics on channels page #412

Implementation

The list of topics will come from making a request to the server, after opening the page.

As a follow-up task, after getting the list for a given channel we might store it (on ChannelStore), and keep it up to date when handling events, so that coming back a second time to this topic-list page for a given channel is fast. For this PR, though, that's out of scope. Instead the list of topics can just live on a StatefulWidget for the page.

We'll need a design for the page. Until that's ready, this issue won't be ready to implement.