fix missing lock info for external module items

test plan:
 1. create a module that requires sequential progress
 2. add an item with a completion requirement
 3. add an external URL and external tool after this item
 4. as a student that has not fulfilled the requirement from
    step 2, use the module items API with include[]=content_details
    to list the module items from step 3.
    - use both /api/v1/courses/X/modules/Y/items and
               /api/v1/courses/X/modules/Y/items/Z
    - these should include lock information for the external URL
      and external tool

fixes CNVS-14190

Change-Id: I5064062b46dfbd1d51b1b1622bf21cc10169face
Reviewed-on: https://gerrit.instructure.com/42622
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>