3.7 regression: Fix the width of captioned images inserted into the visual editor.

Pulls some arithmetic outside of some string concatenation to avoid a nasty uglify.js regression: https://github.com/mishoo/UglifyJS2/pull/330.

props LucP.
fixes #25700 for trunk.

Built from https://develop.svn.wordpress.org/trunk@25921


git-svn-id: http://core.svn.wordpress.org/trunk@25880 1a063a9b-81f0-0310-95a4-ce76da25c4cd