make Canvas::APISerializer use exists? when possible

This query is cheaper to use and preferred in our ActiveRecord query
style guide.

fixes CNVS-12534

Change-Id: Iaa6e7669c0971e71ccbc229f0a6a48c73613e4c5
Reviewed-on: https://gerrit.instructure.com/33578
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Stanley Stuart <stanley@instructure.com>
QA-Review: Stanley Stuart <stanley@instructure.com>