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.
Can we just do 

```
if (oaeAuthInfo.imposter) {
  AuthenticationConstants.events.USER_IMPOSTERED ...
} else {
  AuthenticationAPI.emit(AuthenticationConstants.events.USER_LOGGED_IN ...
}
```