fix: Notification page breakages (#5236)

- Remove the cascading foreign key indexes
- Add migration to clean up existing objects

fixes: #4285

app/jobs/migration/remove_stale_notifications_job.rb

@@ -0,0 +1,19 @@
+# Delete migration and spec after 2 consecutive releases.
+class Migration::RemoveStaleNotificationsJob < ApplicationJob
+  queue_as :scheduled_jobs
+
+  def perform
+    remove_invalid_messages
+  end
+
+  private
+
+  def remove_invalid_messages
+    deleted_ids = []
+    Message.distinct.pluck(:inbox_id).each_slice(1000) do |id_list|
+      deleted_ids << (id_list - Inbox.where(id: id_list).pluck(:id))
+    end
+
+    Message.where(inbox_id: deleted_ids.flatten).destroy_all
+  end
+end