safe nav permissions for attachments
safe nav permissions for attachments

test plan
 - have an attachment with a valid context
 - hit the attachment in the api for public course
 - it should not 500

flag=none
fixes VICE-1189

Change-Id: I43e44b20a8c98997935e27cd06b5731ce085d7a9
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/259893
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>
QA-Review: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>
Product-Review: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms