chore: Delete related notifications if a conversation is deleted (#2592)

db/migrate/20210708140842_remove_notifications_without_primary_actor.rb

@@ -0,0 +1,9 @@
+class RemoveNotificationsWithoutPrimaryActor < ActiveRecord::Migration[6.0]
+  def change
+    deleted_ids = []
+    Notification.where(primary_actor_type: 'Conversation').pluck(:primary_actor_id).uniq.each_slice(1000) do |id_list|
+      deleted_ids << id_list - Conversation.where(id: id_list).pluck(:id)
+    end
+    Notification.where(primary_actor_type: 'Conversation', primary_actor_id: deleted_ids).destroy_all
+  end
+end