Merge pull request #1460 from loic/ObjectMixins.get_queryset

Improved queryset handling and docs for (Single|Multiple)ObjectMixin.
Fixed #12288 -- Added unique validation for INSTALLED_APPS
Fixed test failures introduced by #12288
Fixed test failures on Python 3 - refs #12288
Fixed #9057 -- Added default_permissions model meta option.

Thanks hvendelbo for the suggestion and koenb for the draft patch.
Fixed #19617 -- Refactored form metaclasses to support more inheritance scenarios.

Thanks apollo13 and mjtamlyn for the reviews.