When editing a plain, legacy [gallery] shortcode, ensure we are sorting its...

When editing a plain, legacy [gallery] shortcode, ensure we are sorting its contents using menu_order.

When attachments are sorted, we convert the shortcode to [gallery ids=""], as before.

props koopersmith.
fixes #22608.



git-svn-id: https://core.svn.wordpress.org/trunk@22882 1a063a9b-81f0-0310-95a4-ce76da25c4cd