fix quiz answer serializer loading for cache_classes=false

refs CNVS-13790

the serializers aren't all loaded in development mode, so you can't
rely on them auto-registering themselves. just constantize what the
class name should be.

also fix a module method not being in the correct file, also for
autoloading goodness

Change-Id: Id7da60757c1b7d816fe94e39d57b521e9efb730d
Reviewed-on: https://gerrit.instructure.com/37173
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ahmad Amireh <ahmad@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>