add lock information to module items api

test plan:
 * create a module
 * add an unlocked module item to the module
 * add a locked module item to the module
 * query the module's item list with include[]=content_details
   - the content_details for each item should include:
     - locked_for_user
     - lock_explanation (for locked items)
     - lock_info (for locked items)

fixes CNVS-12296

Change-Id: I68c994ea5d0323e736388e59e48f2d08539f1b1c
Reviewed-on: https://gerrit.instructure.com/33142
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Sterling Cobb <sterling@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>