[TNL-5632] Optimize the single_thread view.

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.

openedx/core/djangoapps/course_groups/cohorts.py

@@ -13,7 +13,7 @@ from django.utils.translation import ugettext as _
 
 from courseware import courses
 from eventtracking import tracker
-from request_cache.middleware import RequestCache
+from request_cache.middleware import RequestCache, request_cached
 from student.models import get_user_by_username_or_email
 
 from .models import (
@@ -485,6 +485,7 @@ def set_course_cohort_settings(course_key, **kwargs):
     return course_cohort_settings
 
 
+@request_cached
 def get_course_cohort_settings(course_key):
     """
     Return cohort settings for a course.

openedx/core/djangoapps/course_groups/tests/test_cohorts.py

@@ -264,7 +264,7 @@ class TestCohorts(ModuleStoreTestCase):
 
     @ddt.data(
         (True, 3),
-        (False, 9),
+        (False, 7),
     )
     @ddt.unpack
     def test_get_cohort_sql_queries(self, use_cached, num_sql_queries):