Add basic versioning history support for master view

as with actual data versioning, we only support Person thus far

tailbone/forms/fields.py

@@ -42,9 +42,8 @@ def AssociationProxyField(name, **kwargs):
                 setattr(self.parent.model, self.name,
                         self.renderer.deserialize())
 
-    def value(model):
-        from rattail.db import model
-        return getattr(model, name, None)
+    def value(obj):
+        return getattr(obj, name, None)
 
     kwargs.setdefault('value', value)
     return ProxyField(name, **kwargs)