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.
I think I would be in favor of creating a util function that pulls the `id`, `resourceSubType`, `visibility` and `previewStatus` from the content object and puts it on the params