add includes for user bio in course user api

for convenience and to reduce number of calls needed to
get all bios for a course

Test Plan:
 * call the courses/:id/users with ?include[]=bio
 * the users' bios should be returned

closes PLAT-804

Change-Id: Ieafd1071b9b31a1bba98472411145c7722549673
Reviewed-on: https://gerrit.instructure.com/45726
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Derek Hansen <dhansen@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>