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 
I believe the tests in that directory are for the files at the `djangoapps/instructor` level and that there aren't tests at the `djangoapps/instructor/view` level.  ` test_enrollment.py` are tests for `djangoapps/instructor/enrollment.py` rather.

Nonetheless, it is rather confusing!  What do you think if I renamed this test to `test_instructor_dashboard_view.py`, or if I created a `view` directory in the `test` directory where I'd break the tests into `test_enrollment.py` and `tests_demographic.py`?  What do you think?