Make redis broker compatible with celery 4.0

The main difference between how instructor-paced and self-paced courses
work with respect to the outline on the backend is how we treat the
hide_after_due attribute on subsections (sequences). Namely, self-paced
courses ignore due dates even if they are specified on the sequence
(for example, by OLX import). If hide_after_due is True in a self-paced
course sequence, we only make it inaccessible after the entire course
ends.

This was tracked as BD-29 and TNL-7262.

fixing language issue for original activation email on sites using mo…

https://openedx.atlassian.net/browse/CRI-217
https://discuss.openedx.org/t/activation-email-in-multiple-languages/2808

Refactor program type enrollment checks

Checking if a user was enrolled in a program type was using the `name`
field which is subject to be translated. This change allows for us to check by
the type's slug which will be constant. This also includes the addition
of api.py files for the course_modes, catalog, and programs apps.

Bumping enterprise to 3.6.8

* Commits:
  Convert InvalidKeyError exception to NotFound

ADR for the role of XBlock

Architecture Decision Record detailing the history of XBlock and
where we're going with it in the platform.

Cleanup unused CONFIG_ROOT variable and dead code.

Add logs in get_problem_responses

Python Requirements Update

* Commits:
  Log and skip empty children when rendering library_content

instead of throwing an exception on None-access.

Fix issues with video transcript panel

[BD-05] [TNL-7308] Add ORA Zipped File Download for Submission Text + Attached Files

This fixes the execption in lms when accessing a blank course via the
front-end-learning MFE.

Allow entitlements to be used past the course has ended but
upgrade deadline is still in future for already enrolled
learners.

PROD-1497

disable xsslint on program_header_view

This fixes a bug where the video transcripts panel would be open on
page load even when no transcripts were present. It also cleans up the
code a bit to remove unused cookies and tries to reduce flickering of
the transcript panel.