Widget Customizer: Move style/script registration to script loader. Update Gruntfile.js.

fixes #27356.

git-svn-id: https://develop.svn.wordpress.org/trunk@27525 602fd350-edb4-49c9-b593-d223f7449a82
Merge branch 'master' into widget-customizer-patches

Conflicts:
	src/wp-includes/class-wp-customize-widgets.php