Firstly, we're now explicitly instructing the comments service to not
return thread responses/comments if the request isn't AJAX.  So, if you
load the URL for a single discussion thread in your browser, this would
be a non-AJAX call and we'll avoid loading the responses for the entire
thread behind-the-scenes.  Big win here for large threads.

Next, we removed a redundant "get threads" call which was also happening
behind-the-scenes.  This call was redundant as the front-end JS also
grabs the thread list when a topic is chosen, so we were making an
extranenous call for no benefit.  Poof, gone!

Finally, we added some caching of database queries that are required to
drive a lot of the permissions/cohorts machinery around discussion.
This will have a minimal effect but introduced a cleaner way to apply
general memoization at the per-request level which will let us further
cache things as we identify them as issues.

Discussion is restricted to the CCX course (there is no sharing of
discussions with the parent course or other CCX instances).

* Renames discussion_module to discussion_xblock
* Moves common/lib/xmodule/xmodule_discussion to openedx/core/lib/xblock_builtin/xblock_discussion

Prevents start date alone from being used to filter out categories in self-paced courses during construction of the category map. ECOM-4017.

Prevents start date alone from being used to filter out categories in self-paced courses during construction of the category map. ECOM-4017.

CCX REST APIs
OAUTH2 authorization for CCX APIs
- oauth2 authorization required for ccx list.
- Course-instructor permission for ccx api endpoint
- Protection for detail view too.

Tests for CCX REST APIs and OAUTH2 authorization

Please note that this is a squshed commit and the work of:
Symbolist, macdiesel, nedbat, doctoryes, muzaffaryousaf and muhammad-ammar

* Fix paver violations to stablize edx-lint update
* Parens, Line2Long
* Fix missing docstrings
* Fix PEP8 issues
* Address PR feedback (thanks @nedbat!)

This reverts commit 4c1c4619.

TNL-1908

Caches all permissions per user per course. Adds caching functionality to has_permission and replaces all instances of cached_has_permission with has_permission.

TNL-1256

TNL-1258

course cohort settings only once.

TNL-1258

TNL-1258

TNL-650

Conflicts:
	lms/djangoapps/django_comment_client/base/views.py
	lms/djangoapps/django_comment_client/tests/test_utils.py
	lms/djangoapps/django_comment_client/tests/utils.py
	lms/djangoapps/django_comment_client/utils.py

This reverts commit 086a2ba7, reversing
changes made to a44132ec.  Some minor
code quality issues were addressed as part of this change.

Conflicts:
	lms/djangoapps/django_comment_client/base/views.py
	lms/djangoapps/django_comment_client/utils.py

When two or more instances of Discussion XBlock were configured
with the same discussion target (Category/Subcategory),
only one of the blocks would be shown on the Course Discussion page.
This was the source of several bugs when trying to edit discussion threads.

This patch adds incrementing numbers to the title of each duplicate
subcategory when rendering the Course Discussion to make sure that
all of the threads are visible in Course Discussion.

simplejson has been removed from future versions of Django (which we will eventually use).

TNL-650

JIRA: TNL-710

IMPORTANT: this commit converts the course_groups
package to using migrations.  When deploying to an
existing openedx instance, migration 0001 may fail
with an error indicating that the CourseUserGroup
table already exists.  If this happens, running
the 0001 migration first, with the --fake option,
is recommended.  After performing this step,
remaining migrations should work as expected.