This specifically enables/disables the underlying comment service client
used to make calls to the service. When disabled, this client will now
throw an exception which can be propagated upwards so that callers can
make the right decision about how to notify users of the error, or
handle retry, etc etc.
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.
