This captures real-time metrics for all of the comment client actions,
segregated by course_id, as well as other small-cardinality fields.
The goal is to be able to detect changes in forum usage, with the goal
of alerting on potential error conditions.
Also improves handling in cases where account creation did succeed, but
comments service user creation did not.
Depends on commit 88abdd8a0b20bc9816f487b25abdea1953f5661d in https://github.com/edx/cs_comments_service
JIRA: FOR-522
CommentClientError now has sane subclasses that are meaningfully
distinct, and each subclass is handled appropriately. Errors raised by
the requests library are no longer handled by turning them into
CommentClientErrors, since there is no meaningful handling we can do,
and this way we will get more visibility into why errors are occurring.
Also, HTTP status codes from the comments service indicating client
error are correctly passed through to the client.
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.
