fix WikiPage#locked_for? to handle sequential modules

test plan:
 - have a course with draft state enabled
 - have a module requiring sequential completion
 - put a wiki page in the module after an item with a requirement
 - as a student that has not completed this requirement,
   attempt to access the page via All Pages
   - should get a message indicating the page is locked

fixes CNVS-11842

Change-Id: Id91cde3677a7c0b6bdcdfb89d565e72f347c3d13
Reviewed-on: https://gerrit.instructure.com/32028
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>