This is follow-up to #1158.

The initial implementation of the topic-list will fetch topics with API calls. Using the data we have, by listening to changes to PerAccountStore and Unreads, we only know about the topic visibility and unreads changes. But not all of its data is up-to-date.

We need to make sure that the data store is specialized for managing what we know about topics, by channels. Some examples of events that need to be handled:

• message events, for new topics;
• update-message events, for moved or (un)resolved topics.

Since the topic-list page orders the topic by the last message sent, the data structure should keep track of the message IDs in each topic.

Ideally, as a follow-up to this, the data store will also keep track of whether previous requests to fetch topics have failed, but that's of lower priority than handling a similar issue with fetch requests errors from MessageListView.fetchInitial.