allow observers to view all module content

test plan:
* make a course with modules with completion
requirements and sequential progress
* log in as an observer in the course
* should not be required to complete content to view it

closes #CNVS-9216

Change-Id: Id2ac39e8d1199bed0e2b17c3d04a58d5c7f4304c
Reviewed-on: https://gerrit.instructure.com/39622
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>