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

spec/components/topic_query_spec.rb

@@ -1020,6 +1020,49 @@ describe TopicQuery do
 
       expect(topics).to eq([])
     end
+
+    context "Calculating minimum unread count for a topic" do
+      before { group.update!(publish_read_state: true) }
+
+      let(:listed_message) do
+        TopicQuery.new(nil, group_name: group.name)
+          .list_private_messages_group(creator)
+          .topics.first
+      end
+
+      it 'returns a positive number when noone has read the last message' do
+        group_message.update!(highest_post_number: 1)
+        TopicUser.create!(user: creator, topic: group_message)
+
+        expect(listed_message.minimum_unread_count).to eq(1)
+      end
+
+      it 'returns 0 when all posts were read' do
+        group_message.update!(highest_post_number: 1)
+        TopicUser.create!(user: creator, topic: group_message, highest_seen_post_number: 1)
+
+        expect(listed_message.minimum_unread_count).to eq(0)
+      end
+
+      it 'returns the minimum number of unread posts when there are more than one user' do
+        new_user = Fabricate(:user)
+        group.add(new_user)
+        group_message.update!(highest_post_number: 3)
+        TopicUser.create!(user: creator, topic: group_message, highest_seen_post_number: 1)
+        TopicUser.create!(user: new_user, topic: group_message, highest_seen_post_number: 2)
+
+        expect(listed_message.minimum_unread_count).to eq(1)
+      end
+
+      it 'returns the minimum number of unread posts when there are more than one user' do
+        new_user = Fabricate(:topic_allowed_user, topic: group_message).user
+        group_message.update!(highest_post_number: 3)
+        TopicUser.create!(user: creator, topic: group_message, highest_seen_post_number: 1)
+        TopicUser.create!(user: new_user, topic: group_message, highest_seen_post_number: 2)
+
+        expect(listed_message.minimum_unread_count).to eq(2)
+      end
+    end
   end
 
   context "shared drafts" do