This completes the work started in https://github.com/edx/edx-platform/pull/19453
to use the LMS login and registration for Studio, rather than Studio
providing its own implementation.

LMS login/registration are being used for the following reasons:
1. LMS logistration properly handles all SSO integrations.
2. A single logistration is simpler to maintain and understand.
3. Allows Studio to work more like all other IDAs that use LMS
logistration.

The original switch to use LMS logistration for Studio also added the
toggle `DISABLE_STUDIO_SSO_OVER_LMS` to provide the community some
additional time for switching. This commit removes this toggle, which
at this point means all deployments will use the LMS logistration.

This change requires sharing cookies across LMS and Studio. Should that
prove to be a problem for certain Open edX instances, there are
discussions of possible alternative solutions.
See https://github.com/edx/edx-platform/pull/19845#issuecomment-559154256

Detailed changes:
* Fix some Studio links that still went to old Studio signin and signup.
* Remove DISABLE_STUDIO_SSO_OVER_LMS feature toggle.
* Remove old studio signin and signup pages and templates.
* Fix url name "login", which had different meanings for Studio and LMS.
* Use the following settings: LOGIN_URL, FRONTEND_LOGIN_URL,
FRONTEND_LOGOUT_URL, and FRONTEND_REGISTER_URL.
* Redirect /signin and /signup to the LMS logistration.
* Add custom metric `uses_pattern_library`.
* Add custom metric `student_activate_account`.
* Add Django Settings to allow /signin, /signup, and /login_post to be
disabled once ready.

This work also relates to ARCH-218 and DEPR-6.

ARCH-1253

Required these be added to CMS since enterprise is installed in the CMS
and there are import failures when these keys don't exist

API docs are now always available, no more toggle to enable them.

[ENT-2191]

- The fields need to be unicode not bytes.  In python 2 it was auto
converting.
- The static url base has been allowed to be a unicode string for a
while.

https://github.com/edx/edx-platform/pull/20645

This introduces:
* A new XBlock runtime that can read and write XBlocks that are persisted using
  Blockstore instead of Modulestore. The new runtime is currently isolated so
  that it can be tested without risk to the current courseware/runtime.
* Content Libraries v2, which store XBlocks in Blockstore not modulestore
* An API Client for Blockstore
* "Learning Context" plugin API. A learning context is a more abstract concept
  than a course; it's a collection of XBlocks that serves some learning purpose.

Some deprecated functionality has been removed:

- Reading data field and transforms being applied in the init() method.
- The source field.
- The source_visible attribute.

Moving rbac settings above plugins

Moving the setting above plugins in common.py for the lms and cms as well

This reverts commit 8c805999.

Removing the Deprecated the 'external_auth' package in favor of 'third_party_auth' which is the current recommendation.

This reverts commit b0b8a76d.

I missed the LMS production settings, and Studio in its entirety.

This is a feature that has been deprecated and can be safely removed.

DEPR-7

The JWT_COOKIES_FLAG was a temporary flag used for rollout of the new
JWT cookies. These are live in Production, so we are removing the flag.

Without this flag, we set JWT cookies during login. However, this
requires an oAuth Client that isn't always available during unit tests.
We introduced a feature flag that is only used for unit tests to
disable setting the JWT cookies. The code explains a bit more why this
solution was selected over adding the oauth client to the database.

ARCH-247

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.

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