WARN when configuring UploadListenerService

[kwin]

Currently the UploadListenerService can be configured for arbitrary paths. Whenever some YAML is then placed in those paths, they are automatically executed with the service user (which is having lots of permissions).
As this can easily lead to situations with privilege escalation (i.e. users can grant themselves or others permissions, even if they should not by just placing a YAML file in a certain directory)

Therefore the listener shouldn't be used in most of the cases and using it should lead to a WARN. Preferably only install hooks or JMX (which require admin permissions) should be used for installing YAMLs.

[kwin]

The same config is being used for https://github.com/Netcentric/accesscontroltool/blob/develop/docs/ApplyConfig.md#startup-hook, though.