Make video_privacy optional

This is not the right way to do fields that only appear for some
proposal types, but is forced by the rest of the code.

We should change this so that forms don't include fields that aren't
relevant to their types.

apps/cfp_review/forms.py

@@ -75,6 +75,7 @@ class UpdateProposalForm(Form):
         ("review", "Do not stream, and do not publish until reviewed"),
         ("none", "Do not stream or record"),
       ],
+      validators=[Optional()],
     )
     needs_laptop = SelectField(
         "Needs laptop",