add submission_versions to user_merge

refs CNVS-16530
closes CNVS-16568
This commit addresses the fact that while the versions table is handled
in user_merge, submission_versions (a proxy, indexable table) is not.

Test plan
- Create two users, A and B
- Enroll users A and B into a course with 3 assignments
- Grade one assignment for user A
- Grade a different assignment for user B
- Grade a third assignment for both users
- As a teacher, make a request to the gradebook history api
(/courses/:course_id/gradebook_history/feed?user_id=:user_id)
for both users
- Note the data returned (scores should be distributed per user as they
  were graded prior)
- Merge user B into user A
- The grades for user A should reflect the grades for user B, as well as
  their original grades

Change-Id: I9557b14875f4276ad18c219a456408a0a959d2ff
Reviewed-on: https://gerrit.instructure.com/43603
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Reviewed-by: Cameron Matheson <cameron@instructure.com>
Product-Review: Josh Simpson <jsimpson@instructure.com>