Move Legacy (in Bundle + Core + settings) related code out from eZ Platform kernel, first making the dependency optional and later stop including it completely.

Requirements:

• Split out EzPublishLegacyBundle to its own repo, with 2 separate namespaces (use PSR-4 to fix autoloading):
  • eZ\Bundle\EzPublishLegacyBundle
  • eZ\Publish\Core\MVC\Legacy
• Remove ezpublish-legacy from platform's composer.json (+ implicit by moving legacyBundle out also from kernel repo)
• Fix legacy leaks
  • Debug toolbar
  • Legacy kernel manual reset on console application
  • legacy_mode (move this setting to LegacyBundle)
• Separate cache clearing integration (new stack cache is cleared by legacy when publishing, needs to be done from new stack while avoiding cyclic cache clearing)

If this is merged before setup wizard work we will need a manual way to install eZ Platform somehow (clean or demo install needs to be defined). Aka minimum: not break BDD which uses demo clean install (no demo content), ideally also not break Demo BDD tests (depends on demo content).