use an active record query for user follows

refs CNVS-7032

simpler to read, and means that the result is an integer, not a string,
regardless of database adapter

Change-Id: Ieab3f59fc44c8667f7df8876bff5f499ba50d69c
Reviewed-on: https://gerrit.instructure.com/28566
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>