Use bootstrap.less file order in Customizer

Appending stylesheets by iterating the `__less` Hash creates an order that is not the same as 'bootstrap.less'.

Some stylesheets like 'component-animations.less' and 'modals.less' have selectors with matching specificity, and so file order decides which style wins. This causes issue #10030 by putting `.fade.in` after `.modal-backdrop.in` and gives the backdrop a higher opacity than intended.

This change uses the Less ordering in 'bootstrap.less' to generate the final stylesheets in the Customizer to make sure customized file ordering matches the distribution file order.

Fixes #10030