[hooks] major refactoring: * they are no "proper" appobject, selected when an event is fired according to its context * new module cubicweb.server.hook containing the Hook class and Operation class * deprecated SystemHook and PreCommitOperation classes * rewrite core server hooks as appobjects in the cubicweb/hooks directory -> deprecates hooksmanager, remove hooks, schemahooks, securityhooks cubicweb.server sub-modules -> new cubicweb.hooks sub-package -> get back to a (somewhat) working state