Customizer: switch to UglifyJS2 (v2.4.13).

The uglify build is generated with `uglifyjs --self -o uglify.min.js`.
Add the jquery check in customizer's output.
Add the license header in the non minified build too.
Run `grunt`.
Merge pull request #13151 from twbs/customizer-uglifyjs2

Customizer: switch to UglifyJS2