[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

# -*- shell-script -*-
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#
# put here information about external resources used by your components,
# or to overides existing external resources configuration
#
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# CSS stylesheets to include in HTML headers
# uncomment the line below to use template specific stylesheet
# STYLESHEETS = DATADIR/cubes.%(cubename)s.css