Feature/encore webpack

[wbloszyk]

Subject

I am targeting this branch, because {reason}.

Closes #{put_issue_number_here}.

Changelog

### Added
- Added some `Class::newMethod()` to do great stuff.

### Changed

### Deprecated

### Removed

### Fixed

### Security

[core23]

Can we close this in favor of #6177?

[wbloszyk]

@core23 more like close #6177 in favor of this.

Add new assets system (encore webpack) is awesome feature and give new posibility (upgrade bootstrap and AdminLTE, add scss). It is all huge change not only in AdminBundle but in whole Sonata. I need to think how to do it in the best way. Probably I will do it in sandbox first, when templates can be override and after that split it to seperate bundles as final/stable webpack releases. Open dev-kit issue will be require. I will do it when i found free time. Now I'm busy.

So lets keep this open.

[SonataCI]

Could you please rebase your PR and fix merge conflicts?

[wbloszyk]

Close in favor of sonata-project/dev-kit#779