fix permission check in users/_name partial

fixes CNVS-16479

the can_manage_admin_users variable -- meant to help control whether the
acting user can edit/become/merge/delete the shown user and also whether
the acting user can edit the shown user's time zone -- was always
truthy, since even an empty array is truthy

test-plan:
 * given Bob is a teacher and Sally a TA in the same course, with Sally
   logged in
   - visit /courses/:that_course/users/:bob
   - Sally should not see the "Edit" link

 * given Bob is a student and Sally a TA in the same course, with Sally
   logged in
   - visit /courses/:that_course/users/:bob
   - Sally should see the "Edit" link
   - after clicking the "Edit" link, Sally should not be able to change
     Bob's time zone.

Change-Id: I429a0752a034cc5ff10962dcbacee747cd6ec650
Reviewed-on: https://gerrit.instructure.com/42774
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>