Updates to Demographics Display

- Gating on feature enabled for Open edX users
- If feature disabled and Insights URL is set, displaying message pointing users to Insights

![screen shot 2014-10-29 at 12 18 44 pm](https://cloud.githubusercontent.com/assets/910510/4829788/2fea4db0-5f89-11e4-861b-6f1f961155cb.png)


@rocha @dsjen @shnayder @mulby @brianhw @lamagnifica 
I plan to activate this functionality by setting `DISPLAY_ANALYTICS_DEMOGRAPHICS` to `False`. That request won't be made until after demographics has been launched in Insights.