Allow import reaction to target group

[fsuter]

Sometimes an import cannot be accomplished with a single configuration. This is (partly) what groups are for. As such, it would be convenient for the import reaction to be able to target a group rather a single configuration.

NOTE: this means that groups must allowed also for non-synchronizable configurations, which is currently not the case (see \Cobweb\ExternalImport\Domain\Repository\ConfigurationRepository::findByGroup()).