Webpack encore and templates upgrade

[wbloszyk]

ATM we have the problem, that we use hard copies of all external assets.

Resolve

1. Support for old external assets
   • move current external assets to seperate place
   • remove it from bundles
   • download it when people use old assets system (someting like phpunit)
2. New assets system - webpack
   • create new sandbox branch for it
   • add webpack for current bootstrap3 templates
   • upgrade templates to bootstrap4

Closes #433, #398.

TODO

SonataAdminBundle

• dependencies
  • Move jquery.confirmExit.js to own files
  • ? jqueryui
  • ? eonasdan-bootstrap-datetimepicker (in its current version)
  • Remove ionicons
  • Remove x-editable (or make it compatible with boostrap 4 or implement own solution)
  • Upgrade admin lte to v3 (check sandbox branch and older prs on sonata admin)
  • Integrate / Remove missing dependencies
  • Upgrade some dependencies (and stop fixing versions on package.json)
• webpack
  • Integrate our code with the webpack encore process
  • Consider integration with webpack encore bundle
• tools
  • Add ESLint
  • Add stylelint
• other
  • Documentation about the frontend assets for contributors
  • Documentation about the frontend assets for users of the bundle
  • Generate the dist files only on release (maybe not necessary)

[wbloszyk]

@core23 @jordisala1991 @greg0ire @VincentLanglet Can you create new sandbox branch 'webpack' from master? I will prepare structure for some build like sonata_admin_v3, sonata_shop_v3, sonata_admin_v4 ... and templates bootstrap3, bootstrap4`.

[greg0ire]

Sure, done.

[jordisala1991]

The work for SonataAdmin is done. Maybe we could use an issue to track what other packages are needed.