Modernise frontend code

[jordisala1991]

Subject

I am targeting this branch, because this is BC break.

Changelog

### Changed
- Change frontend compilation system from Gulp to Webpack
- Change final minified file to `app.js` and `app.css`

[jordisala1991]

Your latest PR on admin bundle might have break some builds @VincentLanglet

[VincentLanglet]

Your latest PR on admin bundle might have break some builds @VincentLanglet

I'm using the modelManager in some methods now, so if it's not set in unit test it will fail indeed.

[VincentLanglet]

#1560

[jordisala1991]

Two things fixed here (besides the full migrations and upgrade of the code to be a little better):

• Composer page now kinda works (not it does not fail on the js part, but rather when saving blocks on the PHP part, to see on another PR)
• Layout is a bit better on Sonata, before it was a bit broken on some parts.

cc @eerison

[VincentLanglet]

Nice job