[skeleton] add pypi classifiers in __pkginfo__ and setup.py (closes #2502156)

[c-c shell -H] add verbosity=0 so we are not asked to confirm everything, as when not using -H

mock ``CWUser.prefered_language()`` on InternalManager objects. The recent split of patches for session refactoring broke this.

[session] fix arguments default value and promote usage of security_enabled as session method. Closes #2481820 One should use session method rather than direct usage of the context manager of the same name. Fix default argument values for consistency with the context manager: when one omit an argument, meaning is "keep the current value", not "disable security".

[session] promote usage of [deny|all]_all_hooks_but session methods rather than hooks_control context manager directly

[web app] move set of status_out into validation_error_handler to ease readability

[workflow test] don't use session.user, subject to internal changes

[db creation test] more testing of db initialization: call build_db_cache and ensure admin user exists

[source synchronization hooks] Fix/enhance system source hooks They are broken if launched during repository initialization (this was not the case yet, but will be soon...). Add additional checks to ensure one doesn't try to store system source config in the database, as it will be ignored in favor of the "sources" file.

[validation error] refactor validation error handling so translation is done on the web side Users should now use cubicweb.validation_error helper function that will activate the feature with other handy behaviours. Also test testing for message in errors should call exception.tr(unicode) before comparing. Using bare ValidationError keep backward compat.