Cluster of priority features focused on message feed, compose, and channel settings

This project focuses on implementing a cluster of high-priority features in Zulip across the message feed, compose box, and channel settings. In the compose box, it introduces reply-without-quoting, collapsed recipient pills for DM conversations, and auto-pillify on blur. In the message feed, it adds always-visible read receipts for DM conversations, multi-message deletion, a "New Messages" divider, move-by-recency for topics, and completes the zulip-icon-at-sign rollout to mention pills. In channel settings, it adds group-level unsubscribe for channels and shows inherited permissions as uneditable pills. The work builds on existing patterns in the Zulip codebase and focuses on shipping well-integrated, user-facing improvements across these areas.