fix django import errors during xmodule_assets

xmodule_assets doesn't pass edx's django settings when it executes
resulting in an import error for xmodules that access django settings
directly or imports code that does.

Updated ProctorPanel to move all code that needs to read from django
settings into __init__. This includes the import of
django.contrib.auth.models.User which automatically tries to read from
django settings on load.