EDUCATOR-341

EDUCATOR-229

EDUCATOR-11

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