Support the set of features related to following topics

[neiljp]

Opening this to track everything that followed-topics involves, which is summarized at
https://zulip.com/help/follow-a-topic#follow-a-topic

Core state tracking & display:

• Ensure that muted topic features are complete (initialize, events, UI) #745
• Extend topic-specific data to support the 'user-topic' concept (default/muted/followed/"unmuted")
• Show & update topic state in UI (topic list, top of message list?, message topic header?)

Notifications:

• Track notification settings locally (initialize + settings events)
• Notifications based on followed topics

Settings changes via Terminal

• Allow adjusting of whether topics are followed via UI (topic popup)

Variant of n key

• Support for cycling through only followed unread topics (N)

The tracking issue in the main repo ended up being zulip/zulip#25673