use context for post_to_sis settings when context is a course
use context for post_to_sis settings when context is a course

refs SIS-2796
fixes CNVS-38095

test plan
 - have a user with no courses
 - it should not error

test plan2:
 - Turn on `Enable new SIS integration settings` feature option,
   check name length setting and require due date setting
 - Navigate to a course
 - Masquerade as the teacher
 - Navigate to the Calendar
 - Create and Assignment event
 - Confirm that the `Sync to #{SIS friendly name}` option is
   present
 - Make the name too long and delete the due date
 - With the sync option checked confirm the validations are shown
   properly
 - Confirm Calendar Assignment Events work correctly via creation
   and editing with the sync option checked/unchecked

Change-Id: I5a0159e879697db3c83d832d7901d3792a457597
Reviewed-on: https://gerrit.instructure.com/119194
Tested-by: Jenkins
Reviewed-by: Brad Humphrey <9233b6f5388d6867a2a7be14d8b4ba53c86cfde2@instructure.com>
Reviewed-by: Nick Houle <972e9d73d0c431cb094492b619fbc612b5441e91@instructure.com>
QA-Review: Gentry Beckmann <f487939f789774cca922ca5b886632de73477ea0@instructure.com>
Product-Review: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>

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