Chunk the notification type backfill migration

Long-term we want that migration to be done entirely in SQL,
but for now this is a hotfix to not cause OOMs on large databases.

This is using a homegrown version of `Repo.stream`, it's worse in
terms of performance than the upstream since it doesn't use the same
prepared query for chunk queries, but unlike the upstream it supports
preloads.

lib/pleroma/migration_helper/notification_backfill.ex

@@ -18,7 +18,7 @@ defmodule Pleroma.MigrationHelper.NotificationBackfill do
       )
 
     query
-    |> Repo.all()
+    |> Repo.chunk_stream(100)
     |> Enum.each(fn notification ->
       type =
         notification.activity