About: Fix the WordPress badge.

`css-clean` (minifies our CSS) has an issue when using multiple backgrounds and background position together. Instead of relying on browser support for multiple backgrounds to display the WP logo as SVG, use the `.svg` CSS class.

Merges [30946] to the 4.1 branch.

Fixes #30735.

Built from https://develop.svn.wordpress.org/branches/4.1@30949


git-svn-id: http://core.svn.wordpress.org/branches/4.1@30938 1a063a9b-81f0-0310-95a4-ce76da25c4cd