Check that the foreign object `from_fields` are a subset of any unique
constraints on the foreign model.

Thanks to Tim for the review.

Prior to this change proxy models reverse fields didn't include the
reverse fields pointing to their concrete model.

Thanks to James Murty for his work on an alternate patch.

This isn't required anymore now that reverse foreign keys
from proxy models are propagated to their concrete model.

Thanks to Anssi for the review.

Thanks Bibhas Debnath for the report and Tim Graham for the review.

The removed test was added in the original select_related() validation
patch (45d4e43d2d25b902e3821b612209afa951a8bcb8), but there doesn't
seem to be any reason for it.

Thanks Claude Paroz for help and review.

Forward port of 86dc4889 from master.

Forward port of 60471c96 from stable/1.9.x.

Fixed on other backends by 134ca4d4.

Thanks Mariusz Felisiak for the solution.

Forward port of f717cb2a from stable/1.9.x.

Thanks Bibhas Debnath for the report and Tim Graham for the review.

Fixed in c9a96075.

Fixed in f403653c.

Thanks Trey Hunner for cleaning the initial patch and Tim Graham for
the review.

This option specifies the level that check command exits with a
non-zero status. Default is ``ERROR``.

The workaround added in fe8484ef
seems unnecessary as the Python bug is fixed in Python 3.4.