This silences one instance of the warning that's being printed in tests
for versions of Django before 2.0:
> RemovedInDjango20Warning: Managers from concrete parents will soon
> qualify as default managers if they appear before any other managers
> in the MRO.
Tests were failing because of:
* Models weren't being imported from their new location.
* PolymorphicManager doesn't work with non-polymorphic models, but
migrations run models' manager, so having MROBase3 with
PolymorphicManager failed.
* Some tests failed due to have assertions were being done (since they
hardcoded expected instance ids). Update tests to use comparisons
similar to #312
Tests were failing on postgres due to foreign keys pointing to missing
tables.
In order for these tables to be present, we need migrations which
declare depending on `content_type`.
Charlie Denton
Krzysztof Gromadzki
Diederik van der Boor
Hugo Osvaldo Barrera