FEATURE: Publish read state on group messages. (#7989)

* Enable or disable read state based on group attribute

* When read state needs to be published, the minimum unread count is calculated in the topic query. This way, we can know if someone reads the last post

* The option can be enabled/disabled from the UI

* The read indicator will live-updated using message bus

* Show read indicator on every post

* The read indicator now shows read count and can be expanded to see user avatars

* Read count gets updated everytime someone reads a message

* Simplify topic-list read indicator logic

* Unsubscribe from message bus on willDestroyElement, removed unnecesarry values from post-menu, and added a comment to explain where does minimum_unread_count comes from

app/serializers/post_serializer.rb

@@ -26,6 +26,7 @@ class PostSerializer < BasicPostSerializer
              :quote_count,
              :incoming_link_count,
              :reads,
+             :readers_count,
              :score,
              :yours,
              :topic_id,
@@ -458,6 +459,13 @@ class PostSerializer < BasicPostSerializer
     can_review_topic?
   end
 
+  def readers_count
+    read_count = object.reads - 1 # Exclude logged user
+    read_count -= 1 unless yours
+
+    read_count < 0 ? 0 : read_count
+  end
+
 private
 
   def can_review_topic?