Add a facility for building stacked configuration models that control the same configuration on a Global, Site, Org and Course level

Content Type Gating: Xblocks that have a graded component cannot be
accessed by audit track users.
  - Caveats:
    - In studio, instructors can set certain xblocks to be available to
      all users, but graded components will default to not being
      available for audit users
    - If a course does not have a verified mode option, all users will
      have access to graded content.

The Waffle Flag: The waffle flag is of for now.
  It's name is: ```content_type_gating.debug```

This Commit Does NOT Include: Displaying for a user WHY they do not have
access to a specific piece of content.  That change will be part of
another PR.

View is combined with user SSO views.

Includes a new version of the view that takes explicit "username" or "email".

OC-4285

Note: this version update is backward incompatible.

ARCH-255

relied on the old configuration values and old way of validating
passwords. Also improved registration page by always showing error
messages rather than hiding them on leaving the field.

From edx-drf-extensions:
1. EnsureJWTAuthSettingsMiddleware: Ensures proper JWT auth settings
   for endpoints.
2. JwtAuthCookieMiddleware: Combines the JWT auth cookie parts into a
   JWT auth cookie.

ARCH-233

Use LOGIN_REDIRECT_WHITELIST to provide a whitelist of
additional domains to which login will now redirect.

ARCH-238

These views and tools don't seem to be enabled and I think it's safe to
say that we have migrated to LMS+CMS.

This upgrades DOT by reverting the revert.

This reverts commit 4d8b9c36

ARCH-180

Add more parameters to COURSE_ENROLLMENT_MODES to allow to display friendly mode name and more options

Fix python tests of PR #18557

PR #18557: Fix Diff Quality test line too long

PR #18557: Fix Diff Quality test unused-variable

Use six.iteritems() in COURSE_MODE_SLUG_CHOICES for future Python 3 compatibility

ARCH-223

ARCH-223

ARCH-220

ARCH-193