[views] implement a custom json error view If the client asked for ``(e)jsonexport`` view, answer him with a json response rather than an error formatted in a big HTML blob. add a custom _requested_vid predicate. In cw 3.21, it will be deprecated in favor of ``match_form_params(vid=('jsonexport', 'ejsonexport'))``

change_attribute_type('CWCache', 'timestamp', 'TZDatetime')
change_attribute_type('CWUser', 'last_login_time', 'TZDatetime')
change_attribute_type('CWSource', 'latest_retrieval', 'TZDatetime')
drop_attribute('CWSource', 'synchronizing')
add_attribute('CWSource', 'in_synchronization')