Use square bounding boxes for default-sized thumbnails

Thumbnails for portrait-orientation images have always been "too big",
especially when displayed in a gallery.  The 'upright' option did not
completely fix the issue.  Using a square bounding box for thumbnails
(and 'framed' images) without an explicit size specifiction provides
a better default appearance.

This also provides a clean syntax for content authored using
Parsoid/Visual Editor, which prefers square bounding boxes.

See:
https://www.mediawiki.org/wiki/Requests_for_comment/Square_bounding_boxes

Bug: 63903
Change-Id: I665d8945843d3b5437a74e376b63c44965590116