Mixpanel tracking

I added an `oae-mixpanel` module that allows us to track various user events with mixpanel.

I also added events for retrieving content, discussion, folder, group and user profiles which can be tracked with mixpanel.
Is visibility part of these update events, or is there a separate update visibility event. If the former, we should store new and old visibility. If the latter, we should track that event as well.