Feature flagged enrollment counts on instructor dashboard.

* DISPLAY_ANALYTICS_ENROLLMENTS feature flag shows or hides the enrollments counts in the instructor 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 
Thanks for the clarification. It is confusing! Generally we try to mirror the unit test directory structure to the code being tested, so I'd go with a new views directory. I'd be fine with keeping this file intact and just moving it into the subdirectory if that's simpler, or splitting it into separate files as you suggest would be fine too.