Add validator for county congressional results

Add a validator, ``validate_2000_primary_congress_county_results``,
that checks that county-level results for congressional offices in the
2000-03-07 Maryland primary have been created.

This validates the ``create_2000_primary_congress_county_results``
transform.

Addresses https://github.com/openelections/core/issues/36
Clean up validate_aggregate_congressional_district_results

Query results using ``contest_slug`` and ``candidate_slug`` to avoid
having to retrieve Contest and Candidate instances.

Add assertion messages.

Addresses https://github.com/openelections/core/issues/36