made course pages invisible to students if tab is hidden in navigation

fixes CNVS-14311

test plan:

- hide pages from navigation in course settings
- as a student, visit courses/1/pages
- should see error saying that the page is not available
- as a student, visit /api/v1/courses/1/pages
- should see message saying page is disabled

Change-Id: I474e2adf1130c3bedd1b1347debd71de0d3ecb50
Reviewed-on: https://gerrit.instructure.com/40702
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Cosme Salazar <cosme@instructure.com>