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.

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

TNL-2864

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

When the thread is closed user can't able to report
the thread and it gives the error. User can now report
thread even it is closed.

TNL-151

Co-authored-by: jsa <jsa@edx.org>

[LMS-2757]

The wrong permission was being checked for pinning/unpinning, so a
sufficiently clever student could pin and unpin threads.

JIRA: FOR-499

fixing unit tests

fixing merge error

fixing xqueue submission issue with unicode url (trial 0.1)

fixing fotmats as commented upon

removing yaml file language selection

Unicode changes to support QRF

removed unnecessary pass in modulestore/init.py

fixing merge error

fixing fotmats as commented upon

removing yaml file language selection

fixing pep8 violations

- fixing pylint violations

pylint violation

fixing line spaces and formats

ignore pylint E1101

remove empty line

fixing pylint violations

 pep8 violations

bulk mail unicode/decode

fix migration error

fix pep8 just to push again

more unicode/decode
Final changes to comments and error messages.

This button has been broken for at least a year, so the code has
suffered from bit rot and should be reimplmemented if the feature is
necessary in the short term (which is unlikely since it has been broken
for so long).

This reverts commit 2df3fe93.

Conflicts:
	common/static/coffee/src/discussion/utils.coffee

This reverts commit acfd0d9f, reversing
changes made to e4194c99.

This reverts commit e7450874.

query the comment service to get their results rather than filtering threads that have
already been loaded.

Allow students to delete threads / respones; prepare for edit view for response comments (but not complete)