TNL-172: Change type of a previously posted post. IMPORTANT: this commit depends on commit 774887b in cs_comments_service.

Fixes TNL-160

Also change server to pass post type through to the comments service.

Co-authored-by: Brian Talbot <btalbot@edx.org>

JIRA: FOR-591

For now, this only includes course id, search terms, and result count.
Information is logged both to the standard logger for near-term analysis
and the event tracker for the longer term. Result count logging requires
commit 02466b1 of cs_comments_service (otherwise it will be null).

Previously, an error was raised if the comments service returned data
including an unexpected field, which unnecessarily complicated the
release path for new features, since the list of allowed fields would
need to be modified before cs_comments_service could be modified, and
only then could edx-platform take advantage of the new CS feature. We
still log a warning if an unexpected field is returned, so we will
still be able to tell if the CS returns a corrupt response.

JIRA: FOR-180

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.

JIRA: FOR-374

The relevant parameters and data are merely passed through between the
front end and comments service.

JIRA: FOR-374

This change requires cs_comments_service version 31ef160 or later. Now
that the /threads endpoint can filter by commentable_ids, use that
instead of the /search/threads endpoint, which does not sort and
paginate correctly.

Bug: FOR-224

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.

This reverts commit 5560a4ad.

This reverts commit 2df3fe93.

Conflicts:
	common/static/coffee/src/discussion/utils.coffee

This reverts commit acfd0d9f, reversing
changes made to e4194c99.

This reverts commit 57427f9a, reversing
changes made to 37621c26.

This reverts commit e4819a1b, reversing
changes made to b6ec41ca.

Now that we're ready to test again, need to revert the revert to put the filtering UI back in.

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.