Improve HippoWebappContextRegistry to replace ServletContextRegistry and make distinctive use of the HippoWebappContext.Type

The HippoWebappContextRegistry is intended to replace the (limited used) ServletContextRegistry.

The only missing (currently used) feature to add is looking up the ServletContext (HippoWebappContext) by its contextpath.

This then also allows removing the HippoWebappListener again, which only was temporarily needed to register a SITE webapp, into the standard HstContextLoaderListener which already is configured in each HST web.xml.

Finally, the latter means that now every HST SITE webapp will get 'registered' (through a ServiceTracker) in the ConfigurationServiceImpl, so this no longer is an error when system parameter use.hcm.sites=false is defined (legacy project structure), but then simply will be ignored.

All this is trivial rewiring, removing the outdated ServletContentRegistry and the temporary HippoWebappListener.