Avoid CS search endpoint for non-search queries

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
Merge pull request #1559 from edx/gprice/cs-commentable-ids-param

Avoid CS search endpoint for non-search queries