touch assignments after moving to new assignment group
touch assignments after moving to new assignment group

test plan:
* have a blueprint course with two assignment groups
* add an assignment
* sync to the child course

* in the blueprint, move the assignment to
 the other assignment group
* delete the assignment group the assignment
 used to belong to
* sync to the child course

* the assignment should still be active in the
 child course

closes #ADMIN-544

Change-Id: I4b907ca35cf3447566845a3fde678343569451f5
Reviewed-on: https://gerrit.instructure.com/132908
Tested-by: Jenkins
Reviewed-by: Mysti Sadler <431d062aff1e8f25416b3a7b732fe6effdb03f9a@instructure.com>
Reviewed-by: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>
QA-Review: Deepeeca Soundarrajan <5bcd4b623f00b7fc2942cd1c65c245b30a48a61f@instructure.com>
Product-Review: James Williams  <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>

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