Feature flagged enrollment counts on instructor dashboard.

* DISPLAY_ANALYTICS_ENROLLMENTS feature flag shows or hides the enrollments counts in the instructor dashboard and legacy dashboard.
* Message directing users to Insights is displayed if DISPLAY_ANALYTICS_ENROLLMENTS is set to false and a ANALYTICS_DASHBOARD_URL is provided.
* Enrollment counts removed from legacy dashboard.

*Legacy Dashboard Enrollments Off*
![legacy-enrollments-off](https://cloud.githubusercontent.com/assets/5265058/4902943/18b481fc-643d-11e4-9ebf-1b98d773e953.png)

*Instructor Dashboard Enrollments Off*
![instructor-enrollments-off](https://cloud.githubusercontent.com/assets/5265058/4902940/18a998fa-643d-11e4-9627-fb79dc785f77.png)

*Instructor Dashboard Enrollments On*
![instructor-enrollments-on](https://cloud.githubusercontent.com/assets/5265058/4902941/18add94c-643d-11e4-83c3-63834e37ef9b.png)

@rocha @clintonb 
Missing doc string