Feature flagged enrollment counts on instructor dashboard.

* enrollments counts removed from legacy dashboard
added tests for instructor_dashboard.py, added docstring for _is_small_course, removed check for settings.ANALYTICS_DASHBOARD_URL when creating dashboard link
fixed quaility errors in instructor_dashboard.py, moved test_instructor_dashboard under tests/views directory
Merge pull request #5838 from edx/dsjen/feature-flag-analytics-enrollments

Feature flagged enrollment counts on instructor dashboard.