Revert "Use square bounding boxes for default-sized thumbnails"

Reverting for further discussion, due to community complaints at https://en.wikipedia.org/w/index.php?title=Wikipedia:Village_pump_%28technical%29&oldid=611154541#Infobox_Image

This reverts commit 7db55341791cb44ca599b84bea707b05de2f227d.

Change-Id: Ieea6224b2a295b5d65bbd486e30cf0573376c7e9