Convert to using Webpack for JavaScript and CSS

[gauthierm]

Experiment to replace SwatHtmlHeadEntry with webpack.

This moves the dependency graph from PHP to JS and means we don't need two dependency graphs (one in PHP and one in YAML).

Proof of concept runs the Swat demo using ES6 modularized Swat widgets.

More work required to:

• modularize YUI 2 and import the parts we need
• incorporate tinymce somehow
• see about bundle splitting, right now everything is one bundle
• expose some more poorly written functions to global NS
• delete IE 6 and 7 hacks

Testing

Swat demo can be used to test:

1. check out this PR
2. cd demo/
3. composer install
4. yarn install
5. yarn build
6. cd www/
7. php -S localhost:8000

[gauthierm]

Issues with YUI 2 with webpack:

• YUI code is written expecting globals. Can resolve this with custom exports/provides webpack config
• Dom has circular reference to Element. Fixed by making a custom loader that patches file on build.
• Element.addListener method uses IE hacks that crash modern browsers. Need to monkey-patch Element some-how. Done again with custom loader.

[gauthierm]

https://app.clubhouse.io/silverorangeinc/story/3024/improve-asset-pipeline-for-legacy-php-sites