Make plugin compatible with new Sylius template events

[gabiudrescu]

This plugin is relying on the Sonata blocks implementation for events in Sylius templates. would you consider upgrading to take advantage of Sylius/Sylius#10997 or rather deprecate this bundle and allow another one from the community to take over this on the long run, once Sonata blocks are removed from Sylius?

[stefandoorn]

Yes, it would be nice to use the new Sylius templates. Recently been doing many updates, mainly with the help of @kgonella.

Obviously open to PR's :-)