Add button/narrow for followed topics

[neiljp]

Similar to the 'mentions' narrow (via button/hotkey), showing messages in which you're mentioned, the 'my messages/threads/responses' (or some related term) narrow would show messages you wrote which have been responded to.

Related main repo links:

• https://github.com/zulip/zulip/issues/6605 [issue: my/sent/recent topics = my messages?]
• https://github.com/zulip/zulip/issues/12309 [issue: replies to my messages?]
• https://github.com/zulip/zulip/pull/14555 [WIP PR: Topics I Follow]

This requires a backend change to enable, as far as I know.

[neiljp]

This feature now has backend support via the following- and unmuted-topics features, which I've opened a tracking issue for at #1485.

I'm leaving this open since the Inbox/Recent-topics(conversations) web view can show a filter by eg. followed topics, as a variant of the 'all messages' variant (ie. excluding muted streams/topics). Web doesn't currently have a standard interleaved view as per this issue.

For moving forward this would need us to think whether to aim for a whole new non-feed view (Inbox/Recent-conversations) and place it there, or also add eg. a 'Followed feed' and 'All feed' as per some options in the web app. Those kinds of views would depend on #1485 to have the data available first in any case.