FIX: Migrate values before adding a not null constraint. (#18940)

Since the migration was added as a post migration, we'll try to add the constraint first, causing a NotNullViolation exception.

This only affects sites that were last deployed more than two days ago.

plugins/chat/db/migrate/20221107034541_make_chat_editor_ids_not_null.rb

@@ -2,6 +2,14 @@
 
 class MakeChatEditorIdsNotNull < ActiveRecord::Migration[7.0]
   def change
+    DB.exec("UPDATE chat_messages SET last_editor_id = user_id")
+    DB.exec(<<~SQL)
+      UPDATE chat_message_revisions cmr
+      SET user_id = cm.user_id
+      FROM chat_messages AS cm
+      WHERE cmr.chat_message_id = cm.id
+    SQL
+
     change_column_null :chat_messages, :last_editor_id, false
     change_column_null :chat_message_revisions, :user_id, false
   end

plugins/chat/db/post_migrate/20221101071135_backfill_editing_user_ids_for_chat_messages_and_revisions.rb

@@ -1,17 +0,0 @@
-# frozen_string_literal: true
-
-class BackfillEditingUserIdsForChatMessagesAndRevisions < ActiveRecord::Migration[7.0]
-  def up
-    DB.exec("UPDATE chat_messages SET last_editor_id = user_id")
-    DB.exec(<<~SQL)
-      UPDATE chat_message_revisions cmr
-      SET user_id = cm.user_id
-      FROM chat_messages AS cm
-      WHERE cmr.chat_message_id = cm.id
-    SQL
-  end
-
-  def down
-    raise ActiveRecord::IrreversibleMigration
-  end
-end