Save autocastDelay as a setting on User re #627

This works but:
1. Currently defaults for this are set in client https://github.com/codecombat/codecombat/blob/master/app/views/play/level/tome/cast_button_view.coffee#L37-40. Should this continue to be the case? Or should a default be set in the schema?
2. Should I be handling the possibility of the save calling failing in some way? I've seen this elsewhere, just not sure what the appropriate action would be here.