Restore but deprecate Mezzanine's get_user_model()
Restore but deprecate Mezzanine's get_user_model()

This wasn’t actually defined before, simply imported from Django. We
define it here in order to provide a deprecation warning for anybody
importing it from this file.

ryneeverett/mezzanine,christianwgd/mezzanine,Cicero-Zhao/mezzanine,douglaskastle/mezzanine,wbtuomela/mezzanine,eino-makitalo/mezzanine,eino-makitalo/mezzanine,wbtuomela/mezzanine,wbtuomela/mezzanine,Cicero-Zhao/mezzanine,gradel/mezzanine,stephenmcd/mezzanine,ryneeverett/mezzanine,stephenmcd/mezzanine,spookylukey/mezzanine,douglaskastle/mezzanine,jerivas/mezzanine,christianwgd/mezzanine,viaregio/mezzanine,readevalprint/mezzanine,stephenmcd/mezzanine,vladir/mezzanine,ryneeverett/mezzanine,jerivas/mezzanine,dsanders11/mezzanine,readevalprint/mezzanine,sjuxax/mezzanine,vladir/mezzanine,readevalprint/mezzanine,molokov/mezzanine,frankier/mezzanine,sjdines/mezzanine,gradel/mezzanine,frankier/mezzanine,viaregio/mezzanine,molokov/mezzanine,frankier/mezzanine,promil23/mezzanine,jerivas/mezzanine,christianwgd/mezzanine,sjdines/mezzanine,sjuxax/mezzanine,vladir/mezzanine,dsanders11/mezzanine,dsanders11/mezzanine,promil23/mezzanine,spookylukey/mezzanine,promil23/mezzanine,spookylukey/mezzanine,douglaskastle/mezzanine,viaregio/mezzanine,sjdines/mezzanine,sjuxax/mezzanine,gradel/mezzanine,eino-makitalo/mezzanine,molokov/mezzanine