Resources associations for MutlipleChoiceAnswer and OpenResponseAnswer

specs, routes, models, controllers.

When a student answers questions in an activity, their responses are
saved back to the server so that they can be seen when the student
returns to the activity.

part of [#48276409]
https://www.pivotaltracker.com/story/show/48276409
Module Embeddable : factor-out 'table_name_prefix'