[session] move context manager logic in the security context manager
The `init_security` and `reset_security` method have no other user that the context manager itself. Having dedicated public function is not useful.
from cubicweb.schemas.base import in_group, CWSourceSchemaConfig
# copy __permissions__ to avoid modifying a shared dictionary
in_group.__permissions__ = in_group.__permissions__.copy()
in_group.__permissions__['read'] = ('managers',)
cw_for_source = CWSourceSchemaConfig.get_relation('cw_for_source')
cw_for_source.__permissions__ = {'read': ('managers', 'users'),
'add': ('managers',),
'delete': ('managers',)}