Also, tweak the un_sys_path.sh script.

Removed most of the deprecated shoppingcart app, leaving just enough to allow us to cleanly remove the related database tables later. Also removed the relevant Django settings that weren't in use elsewhere.

We also split the feature toggle and settings docs in different pages.

We introduce the documentation of django settings via code annotations.
This will allow us to produce a human-readable documentatio of all Open
edX settings.

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

We introduce a new documentation target, where we use the featuretoggles
Sphinx extension from code-annotations to generate human-readable
documentation of feature toggles in edx-platform. The annotation report
is generated on-the-fly based on the standard feature toggle
configuration file in code-annotations.

In addition, we add new doc.in & doc.txt requirement files that will be
pip-installed by readthedocs to generate the documentation targets.

* Moving plugins infrastructure to edx-django-utils
This PR extracts the code that enables plugins in edx-platform and puts it in edx-django-utils. This is done to allow other IDAS to add plugin functionality.

Introduce an Architectural Decision Record about the boundaries
and responsibilities of Studio (Content Authoring) and the LMS
(Learning Experience). These are not new ideas. This is just
explicitly writing down the many local decisions that have been
made for years (Block Transformers, edx-when, grading, etc.), and
showing how they add together to a set of high level principles.

I hope to eventually have docs describing Studio/LMS's relation
to other subdomains in Open edX.

* fix old migrations
* fix a test
* fix enable-migrations cmd argument
* enhance verbose logging to log migrations
* update testing doc with help

- Fix indenting.
- Small wording fixes.

- Add ADR describing Plugin Contexts
- Remove app-specific constants from framework-level code
- Add dashboard constants to student app with README

[TNL-7051] Clicking a video XBlock's fullscreen button now takes the video fullscreen instead of full window.

Gracefully fallback to full window if fullscreen apis are absent

This will remove imports from __future__ that are no longer needed.

https://docs.python.org/3.5/library/2to3.html#2to3fixer-future