fix visibility of sidebar buttons

test plan:
 - with draft state disabled
 * as a teacher, navigate to the home page of a published course
   - 'New Announcement' should be visible
   - 'Course Setup Checklist' should be visible
 * as a student, navigate to the home page of a published course
   - 'New Announcement' should *not* be visible
   - 'Course Setup Checklist' should *not* be visible

 - with draft state enabled
 * as a teacher, navigate to the home page of a published course
   - 'New Announcement' should be visible
   - 'Course Setup Checklist' should be visible
 * as a student, navigate to the home page
   - 'New Announcement' should *not* be visible
   - 'Course Setup Checklist' should *not* be visible
   * change course homepage to something other than the default (feed)
	   - 'View Course Stream' should be visible

fixes CNVS-10615

Change-Id: I598ede4d5ca12de42bc865f59af5c795d5658053
Reviewed-on: https://gerrit.instructure.com/28868
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>