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]