base_concrete_model: accept model class argument.
base_concrete_model: accept model class argument.

Previously only model instances were accepted but now model classes can
be passed alternatively.

sjdines/mezzanine,readevalprint/mezzanine,jerivas/mezzanine,dsanders11/mezzanine,sjdines/mezzanine,jerivas/mezzanine,wbtuomela/mezzanine,sjuxax/mezzanine,ryneeverett/mezzanine,wbtuomela/mezzanine,frankier/mezzanine,frankier/mezzanine,molokov/mezzanine,readevalprint/mezzanine,Cicero-Zhao/mezzanine,molokov/mezzanine,sjuxax/mezzanine,jerivas/mezzanine,molokov/mezzanine,sjdines/mezzanine,stephenmcd/mezzanine,christianwgd/mezzanine,ryneeverett/mezzanine,christianwgd/mezzanine,frankier/mezzanine,dsanders11/mezzanine,stephenmcd/mezzanine,readevalprint/mezzanine,christianwgd/mezzanine,ryneeverett/mezzanine,stephenmcd/mezzanine,sjuxax/mezzanine,Cicero-Zhao/mezzanine,wbtuomela/mezzanine,dsanders11/mezzanine