fix users admin merge autocomplete box

test plan:
* as an account admin, visit /users/:id/admin_merge
* the "Find a User" autocomplete box should work
 (note: the minimum search length is 4 characters)

fixes #CNVS-11476

Change-Id: I74095750a23085d63b814d80e40dacc62133ff7c
Reviewed-on: https://gerrit.instructure.com/34224
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>