add is_public to accounts api listing courses

Test Plan:
 * use the account api's courses endpoing
 * the is_public property should be there

closes PLAT-797

Change-Id: Icf6f26bbff6ca0da0533700bb0cfd264fd833c12
Reviewed-on: https://gerrit.instructure.com/45483
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Humphrey <brad@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>