Internally, Django represents an order_by clause with an OrderBy

instance, and this can contain nulls_first= and nulls_last=

settings. These settings affect comparisons, but don't show up in the

repr() output, making it hard to debug. They can also support

subqueries, which need to be handled just like we do for other clauses.
This change adds explicit comparisons and formatting for order_by to

both show any nulls_* attributes and to store subquery information for

comparison when in use by the query.

All unit tests pass in all test environments.
Review Board and Djblets unit tests pass.