Make type check for self.code a little more robust, give default val for mmlans so it's never undefined

This reverts commit 66889b87, reversing
changes made to 0dd9c24e.

This was merged too early--it's still missing the filtering and display-your-cohort changes.
Will need to revert this revert later.

Change unread to read; makes some things clearer both on the comment service and in the django client

use new capa_alert div for error messages, clean up debugging

Also fixed up some random bugs with content_info, and changed pagination code a little. Needs comments service update for the subscribed threads API call.

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

Requires cs_comments_service to have the field default_sort_key on users.

[Hopefully fix #34992235]