move quiz into quizzes namespace as a precursor to enginifying

fixes CNVS-10679
this commit refactors quiz into a quizzes namespace. it contains various
shims to facilitate the data migration of polymorphic relationships
('Quiz' -> 'Quizzes::Quiz').  JIRA contains several tickets linked to
the above tickets in regards to removing these shims after the data
migration, as well as the strategies on reverting the shims once the
data migration is complete.
Change-Id: I30c566d60a87af6ee83e9d0041fdcb909ead6a89
Reviewed-on: https://gerrit.instructure.com/28573
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Myller de Araujo <myller@instructure.com>
Product-Review: Josh Simpson <jsimpson@instructure.com>