For cases where we don't need or want the responses on a thread at all,
like getting some metadata about a thread, we should be able to request
it without the responses.  This exists in the ThreadPresenter but didn't
exist as an option that could be passed in the API call itself.

We've added this as an option now -- with_responses -- that defaults to
true, which preserves the existing behavior but does in fact allow
toggling.

This gives us a backwards-compatible upgrade path to allow changes to be
made the LMS so that requests which don't need the responses, or do need
the responses, can be explicit about what they want.