Fixed issue #9171 : Unable to subscribe to event

Dev : Target not anymore needed for auth plugins events, every auth plugin knows if he has to process event
Dev : This way, auth events (i.e. afterLogout) can be processed also by other plugins
Dev : Method afterLoginFormSubmit can be in parent class (preventing code duplication)
Thanks Denis.

Currently, Authdb and AuthLDAP plugins are subscribed and code is exactly the same. Authwebserver is not subscribed.