define exportable attributes/associations on models

instead of passing an :exportable option to
ActiveRecord::Associations, simply define a constant
on the class containing exportable associations and
attributes. This is due to :exportable breaking
ActiveRecord, and we can't simply monkey-patch in
config/initializers because models are included in
migrations before the initializers are run

Change-Id: I11f1a6b4570c397d8e01010c517bc6efdac7afca
Reviewed-on: https://gerrit.instructure.com/33235
Reviewed-by: Braden Anderson <banderson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Anthus Williams <awilliams@instructure.com>
QA-Review: Anthus Williams <awilliams@instructure.com>