When editing a [gallery], menu_order sorting should be used

[nacin]

Initially reported by devinreams via #22607. When editing a gallery created with simply the [gallery] shortcode, menu_order sorting is not used, which means they end up in a different order than they show on the frontend.

The default "orderby" for the gallery shortcode is menu_order ID, both ascending.

[koopersmith]

Patch adds capability to sort queries by menu_order ID by adding the menuOrder property to Attachment models. Any gallery that does not specify any ids and does not specify a different orderby attribute will use menu order sorting by default. Also, the "uploaded to this post" filter is now sorted by menu order.

[devinreams]

I confirmed the following:

1. Editing an existing gallery with a manual sort order shows those in the proper order
2. Changing the sort order, aka updating the gallery from "Visual" view, still updates the text shortcode to include "ids" parameter
3. The "Uploaded to this post" filter shows the proper order (the manual sort order)

[ryan]

Same here. Looks good.

[nacin]

In 22882:

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.

[nacin]

#22758 was marked as a duplicate.