Replace Sprockets with Webpacker for JS and Sass

Summary

Replace sprockets usage with webpacker for JS and Sass.

Improvements

We'll be able to benefit from webpacker for handling all the assets.

Converting Sass is implemented https://gitlab.com/gitlab-org/customers-gitlab-com/merge_requests/529, this issue will focus on JS.

Risks

Involved components

Refactoring will change:

• app/assets/javascripts/
• app/assets/stylesheets/
• app/javascript/packs/
• app/views/layouts/application.html.haml
• app/views/layouts/simple.html.haml
• app/views/zuora_callbacks/payment_method.html.haml
• vendor/assets/

Optional: Intended side effects

• Extract Zuora events from payment_method (already implemented in this commit).
• Replace files in vendor/assets/ with proper npm module alternative.