Limit types mappable to Responses.

Per [72684780](https://www.pivotaltracker.com/story/show/72684780).  Users not seeing Objectives and other types that get mapped to responses.  They actually should not have been allowed to map them in the first place; the problem was that the mapping list was not correctly excluding those types.  Corrected by replacing the `business_objects` mixin with the subset that captures the mappable objects.

This still captures the Response types, which should not be permitted so `exclude_option_types` is also extended to surgically reject them.