fix: hopefully fix startup continuum bug per 'active_history' models

only have one model in the wild (SpecialOrder) which uses
active_history and it probably shouldn't really..  it was written a
long time ago and would need some refactoring.

but for now we just need things to work, and this seems to do it
although i have no idea why

rattail/db/config.py

@@ -96,7 +96,17 @@ def configure_versioning(config, force=False, manager=None, plugins=None, **kwar
         # TODO: is this the best way/place to confirm versioning?
         app = config.get_app()
         try:
+            # TODO: not sure why but if we don't load app.model here,
+            # for some reason the alembic upgrade command may fail, if
+            # any app models include the 'active_history' feature?!
+            model = app.model
+
+            # but when confirming if versioning is working, we
+            # definitely need to check a "native" model and not
+            # anything from app.model, which may use a totally
+            # different base class with no versioning (i.e. wutta)
             from rattail.db.model import User
+
             configure_mappers()
             transaction_class = continuum.transaction_class(User)
             config.versioning_has_been_enabled = True