Per-channel topics_policy, to replace mandatory_topics

[gnprice]

The Zulip server now has a per-channel version of the mandatory_topics setting. We should start consuming it.

References

• Server PR: settings: Add a new topics_policy realm setting. zulip#34897
• #api design thread: #api design > general chat api changes @ 💬
• Docs changes (as screenshot): #api design > general chat api changes @ 💬