[GeneratorBundle] Add webpack encore - keep Groundcontrol as option

[dbeerten]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Fixed tickets

TODO:

• Check admin JS & CSS
• Check if Webpack encore setup works (with and without demosite flag)
• Check if Groundcontrol setup still works (with and without demosite flag)
• doc install composer require symfony/webpack-encore-bundle
  added symfony/webpack-encore-bundle to cms-skeleton: Add webpack-encore-bundle cms-skeleton#9
• update stylelint & eslint to latest versions (incl configs)
• adjust cookie bundle

BLOCKED BY:

• Add encore_entry_exists() twig functions to check if entrypoint has files symfony/webpack-encore-bundle#147
• with Groundcontrol setup, the encore_entry_exists twig function is not available and results in a render error: [AdminBundle] Handle possible missing encore_entry_exists twig function in legacy groundcontrol FE setup #3074

Feedback?

• Deleted the styleguide for now, should we keep it?
  We can't use Webpack Encore to build the assets as it is not rendered by Symfony
• By default, the cms-skeleton installs the symfony/webpack-encore-bundle package. When Groundcontrol is prefered, this package needs to be removed. Where should we document this? (also: by removing this package, the package.json gets deleted)

[acrobat]

Blocked by: symfony/webpack-encore-bundle#147 for optionally allowing to override admin js/css

[acrobat]

It's maybe a good idea to switch from public/frontend to public/build as the base output dir when using webpack encore? As the webpack encore bundle recipes default adds the build dir to gitignore

https://github.com/symfony/recipes/blob/22ddf06812d17b39ba291eb6aab8ec2bfd0abd3a/symfony/webpack-encore-bundle/1.9/manifest.json#L14