move QuizSerializer into quizzes namespace

closes CNVS-11591

Change-Id: Id33551d07f76fbbcb74612ca5bc2c32a7846801c
Reviewed-on: https://gerrit.instructure.com/31226
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Stanley Stuart <stanley@instructure.com>