fix count call on mysql

mysql doesn't all count(distinct users.*), so use count(distinct user.id)
instead

refs CNVS-7032

Change-Id: I8cc068581ec78d1482ece3502a7253cf5c7dec88
Reviewed-on: https://gerrit.instructure.com/28610
Reviewed-by: Anthus Williams <awilliams@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>