export account using open_data_export

open_data_export adds an option called exportable
to AR associations. If set to a truthy value, that
association is included in exports. This commit sets
this value (inside a CANVAS_RAILS2 guard) for
associations we want open_data_export to include

Change-Id: Idf8163f3139aafbf34e4e78b94dea6be87bd1cd2
Reviewed-on: https://gerrit.instructure.com/30308
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Anthus Williams <awilliams@instructure.com>
QA-Review: Anthus Williams <awilliams@instructure.com>