FIX: Delete associated notifications when trashing chat messages. (#20144)

Deleting a message with a mention doesn't clear the associated notification, confusing the mentioned user.

There are different chat notification types, but we only care about `chat_mentioned` since `chat_quoted` is associated with a post, and `chat_message` is only for push notifications.

Unfortunately, this change doesn't fix the chat bubble getting out of sync when a message gets deleted since we track unread/mentions count with an integer, making it a bit hard to manipulate. We can follow up later if we consider it necessary.

plugins/chat/app/controllers/chat_controller.rb

@@ -260,13 +260,9 @@ class Chat::ChatController < Chat::ChatBaseController
   def delete
     guardian.ensure_can_delete_chat!(@message, @chatable)
 
-    updated = @message.trash!(current_user)
-    if updated
-      ChatPublisher.publish_delete!(@chat_channel, @message)
-      render json: success_json
-    else
-      render_json_error(@message)
-    end
+    ChatMessageDestroyer.new.trash_message(@message, current_user)
+
+    head :ok
   end
 
   def restore

plugins/chat/app/models/chat_mention.rb

@@ -3,7 +3,7 @@
 class ChatMention < ActiveRecord::Base
   belongs_to :user
   belongs_to :chat_message
-  belongs_to :notification
+  belongs_to :notification, dependent: :destroy
 end
 
 # == Schema Information

plugins/chat/app/services/chat_message_destroyer.rb

@@ -11,6 +11,18 @@ class ChatMessageDestroyer
       end
   end
 
+  def trash_message(message, actor)
+    ChatMessage.transaction do
+      message.trash!(actor)
+      ChatMention.where(chat_message: message).destroy_all
+
+      # FIXME: We should do something to prevent the blue/green bubble
+      # of other channel members from getting out of sync when a message
+      # gets deleted.
+      ChatPublisher.publish_delete!(message.chat_channel, message)
+    end
+  end
+
   private
 
   def reset_last_read(message_ids)