FEATURE: Publish read state on group messages. (Originally introduced in #7989) (#8025)

* Revert "Revert "FEATURE: Publish read state on group messages. (#7989) [Undo revert] (#8024)""

This reverts commit 36425eb9f0.

* Fix: Show who read only if the attribute is enabled

* PERF: Precalculate the last post  readed by a group member

* Use book-reader icon instear of far-eye

* FIX: update topic groups correctly

* DEV: Tidy up read indicator update on write

app/controllers/post_readers_controller.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+class PostReadersController < ApplicationController
+  requires_login
+
+  def index
+    post = Post.includes(topic: %i[allowed_groups]).find(params[:id])
+    read_state = post.topic.allowed_groups.any? { |g| g.publish_read_state? && g.users.include?(current_user) }
+    raise Discourse::InvalidAccess unless read_state
+
+    readers = User
+      .joins(:topic_users)
+      .where('topic_users.topic_id = ? AND COALESCE(topic_users.last_read_post_number, 1) >= ?', post.topic_id, post.post_number)
+      .where.not(id: [current_user.id, post.user_id])
+
+    readers = readers.map do |r|
+      {
+        id: r.id, avatar_template: r.avatar_template,
+        username: r.username,
+        username_lower: r.username_lower
+      }
+    end
+
+    render_json_dump(post_readers: readers)
+  end
+end