Allow extra fields returned from comments service

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 is a continuation of #3291; the commit has been rebased, and _update_attributes has a new name and signature that clarify its purpose.

@jimabramson