[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.

common/test/acceptance/tests/discussion/test_discussion.py

@@ -435,22 +435,6 @@ class DiscussionTabMultipleThreadTest(BaseDiscussionTestCase):
         view = MultipleThreadFixture(threads)
         view.push()
 
-    def test_page_scroll_on_thread_change_view(self):
-        """
-        Check switching between threads changes the page focus
-        """
-        # verify threads are rendered on the page
-        self.assertTrue(
-            self.thread_page_1.check_threads_rendered_successfully(thread_count=self.thread_count)
-        )
-
-        # From the thread_page_1 open & verify next thread
-        self.thread_page_1.click_and_open_thread(thread_id=self.thread_ids[1])
-        self.assertTrue(self.thread_page_2.is_browser_on_page())
-
-        # Verify that the focus is changed
-        self.thread_page_2.check_focus_is_set(selector=".discussion-article")
-
     @attr('a11y')
     def test_page_accessibility(self):
         self.thread_page_1.a11y_audit.config.set_rules({