Add feedbackify support

I've made this configurable as it depends on the UI whether a class is needed or not. E.g. we need an extra class for Avocet to add extra padding so that the feedback widget doesn't overlap with the site content.

As an alternative we could hardcode it to always add something like `.oae-feedbackify` to the body when the widget is shown but I'd rather have it configurable as it will make whoever is configuring this aware that this functionality is available as opposed to automagically doing something.

Personal opinion though, could be either.