Remove reference to __v_ prefix

[matthew-white]

As of #1024, the UserPreferences object creates a proxy around a shallow reactive object whereby the proxy gates access to the reactive object's properties. If something tries to read a property that isn't associated with a known user preference, the proxy throws an error. However, Vue seems to be trying to read properties that start with __v_. That seems to be something internal to Vue, not part of the Vue API. For now, the UserPreferences proxy explicitly allows __v_ properties to be read. However, since __v_ is undocumented, it'd be ideal if we could find a way to avoid this reference to it.