Compatibility with mozilla-django-oidc==2.0.0

[stevenbal]

Look into required changes to fix incompatibility (https://github.com/mozilla/mozilla-django-oidc/compare/1.2.4...2.0.0)

Issues:

• [x] SessionRefresh requires workaround for __init__ because it attempts to load settings on initialization instead of runtime
• [x] Missing fields that should be added to config model:
  • OIDC_USE_NONCE
  • OIDC_NONCE_SIZE
  • OIDC_STATE_SIZE
  • OIDC_EXEMPT_URLs