feature request: Support topic hierarchy (or "topic groups")

[a-lunev]

Would you please consider supporting topic hierarchy.
Channels are to group messages along user groups.
Topics are to group messages along a topic. However, it would be great to create an hierarchy of topics (main broad topics, and more detailed sub-topics inside of each main topic).

[showell]

One observation is that this feature seems to particularly resonate with folks who are using automated integrations that post topics to the stream. More context here:

https://chat.zulip.org/#narrow/stream/2-general/topic/grouping.20topics/near/890337

[zulipbot]

Hello @zulip/server-misc members, this issue was labeled with the "area: topics" label, so you may want to check it out!