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`.
move call for removeBackdrop() to backdrop()

Closes #11958 by rebasing & merging it
Merge pull request #13151 from twbs/customizer-uglifyjs2

Customizer: switch to UglifyJS2
grunt dist
add missing operators to some JSCS rules that we already comply with
Update html5shiv to v3.7.1.
Use the new respond.js URL.
spelling fixes in Carousel unit tests