fix sharding bug in User.update_account_associations

The method had a bug where if you passed a list
of user IDs (as opposed to User objects), it would
attempt to execute only on the current shard.

Fixes CNVS-10099

Test plan: specs should pass

Change-Id: I7a6faa20ef5156235af721b7e69264b47cd38a65
Reviewed-on: https://gerrit.instructure.com/27758
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Anthus Williams <awilliams@instructure.com>