Improve validation for Person field of User form

otherwise if user enters e.g. "John Doe" but does *not* select an
autocomplete result, then "John Doe" will be submitted as-is to the
server, which then tried to write that directly to
``users.person_uuid`` column in the DB, resulting in error

tailbone/views/users.py

@@ -164,6 +164,17 @@ class UserView(PrincipalMasterView):
         if query.count():
             raise colander.Invalid(node, "Username must be unique")
 
+    def valid_person(self, node, value):
+        """
+        Make sure ``value`` corresponds to an existing
+        ``Person.uuid``.
+        """
+        if value:
+            model = self.model
+            person = self.Session.query(model.Person).get(value)
+            if not person:
+                raise colander.Invalid(node, "Person not found (you must *select* a record)")
+
     def configure_form(self, f):
         super(UserView, self).configure_form(f)
         user = f.model_instance
@@ -188,6 +199,7 @@ class UserView(PrincipalMasterView):
                 people_url = self.request.route_url('people.autocomplete')
                 f.set_widget('person_uuid', forms.widgets.JQueryAutocompleteWidget(
                     field_display=person_display, service_url=people_url))
+                f.set_validator('person_uuid', self.valid_person)
                 f.set_label('person_uuid', "Person")
 
         # person name(s)