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/spec/requests/chat_controller_spec.rb

@@ -611,7 +611,7 @@ RSpec.describe Chat::ChatController do
     end
 
     before do
-      ChatMessage.create(user: user, message: "this is a message", chat_channel: chat_channel)
+      ChatMessage.create!(user: user, message: "this is a message", chat_channel: chat_channel)
     end
 
     describe "for category" do