[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