EZP-22210: Implement legacy bundles

Required by https://github.com/ezsystems/ezpublish-kernel/pull/719.

- Injects the kernel into `EzPublishLegacyBundle()`
- Adds the legacy bundle install script to `composer post-(update/install)-cmd`
- Adds `NgsymfonytoolsBundle()` to registered bundles (required by the CommentsBundle)